In the online detection of total phosphorus for water quality, due to the limitation of measuring accuracy of micro-spectrometer and some external interference, the stability of spectral data is damaged so that the detection stability and precision is affected. Kalman filter and non-linear regression method are used to deal with spectral data on the basis of ultraviolet spectrophotometry, and then the relationship equation between absorbance and concentration is established. Thus the content of total phosphorus can be calculated. The method can improve the stability and precision of online detection. The problem of stochastic disturbance and system error can be solved very well. It can be used for real-time monitoring of total phosphorus.