An automatic fast determination method of nitrite in seawater was developed by optimizing the previous self-research of sequential injection analysis (SIA), combining our self-developed Z-type liquid waveguide capillary cell (LWCC) flow cell and tubing-looper, using spectrophotometry and incomplete chromogenic reaction to accomplish the automatic fast determination and make its measurement process more suitable for in-situ analysis and monitoring. The heart of the injection technology is a high-precision syringe pump and a multiposition valve (MVP). Cooperating with MVP, the syringe pump inhales samples and reagents in a holding coil in sequence, and then reversely pushes the mixed solution to the mixing coil and an incomplete chromogenic reaction occurred during this period. The syringe pump finally slowly pushes the mixed solution through the Z-type LWCC flow cell, meanwhile, the absorbance changes of flow solution is detected by a spectrometer, and the nitrite concentration of sample is acquired with Lambert-Beer law. Several key parameters of the fast detection method, such as incomplete chromogenic reaction time, flow rate of mixed solution during detection and salinity were analyzed for stable and fast analysis purposes. The study on the incomplete chromogenic reaction shows that the relative standard deviations (RSDs) of absorbance measurement results are all less than 1.64% within 10～60 s reaction time, indicating the chromogenic reaction time of 10~60 s has no effect on the fast detection method, therefore, 10 s is selected as the fast detection method chromogenic reaction time. The research of mixed solution’s flow rate during detection shows that the flow rate has a large effect on the absorbance detection. Rapid flow rate influences the detection instability, and slow flow rate is not conducive to the fast detection. The stability and repeatability of absorbance measurement results are analyzed under the speeds of 10, 11.6, 13 and 15 μL·s-1, which are relatively stable on absorbance measurement on the basis of experimental verification. The analysis results indicate that the linearity under the above four flow rates are all good, so the fastest speed 15 μL·s-1 is selected for the fast detection method. The absorbance changes of three different concentrations of nitrite (150, 250, 350 μg·L-1) in the 0～35 salinity range are analyzed to verify the sensitivity of this fast detection method to salinity and the adaptation to freshwater and the wide range of seawater. The RSDs are 1.39%, 2.03% and 1.28% respectively, indicating that salinity has no effect on this method. The RSDs measured of parallel 80, 150, and 250 μg·L-1 nitrite standard solutions for 11 times are 2.13%, 1.07% and 1.83% respectively, indicating that this fast detection method has a good precision. The detection limit of this method acquired by taking 10 parallel samples of blank samples is 37 μg·L-1 (about 0.5 μmol·L-1). In order to verify the credibility, the standard curves of same batch nitrite standard solutions are made by using the fast detection method in this paper and the standard method in “Specifications for oceanographic survey”. The R2 of above two methods are both greater than 0.999, and the linear regression equation of the measurement data obtained by the two methods of same concentration sample is y=1.046 1x-0.005 7 with the R2=0.999 6, which shows that the results of the two methods are highly consistent, further verifying the feasibility and reliability of the fast detection method in this paper.The determination rate of this method is up to 50 samples·hour-1. Compared with the traditional manual detection method and flow injection analysis method, the nitrite fast detection method in this paper shortens the time -consumed from a dozen minutes to a minute or so, reduces the sample and reagents consumption in the entire detection process. The fast detection method has a good repeatability, and the whole measurement process is fully automated, and the operation is simpler and more intelligent, which avoids the error caused by manual intervention and makes the nutrients online and in-situ detection system based on spectrophotometry more compact, fast and low-consumption, which is more suitable for in-situ and long time monitoring. The fast detection method in this paper is applicable to other seawater nutrients as long as it is slightly adjusted, having a wide range of applications and good application prospects.