UV-Vis （Ultraviolet and visible） spectrometry was used to measure the absorbance of seawater with different concentrations relative to pure water. In order to meet the requirements for high-precision detection of seawater absorbance, the inherent background signal of the instrument and the difference between the sample containers were used to detect the absorbance of seawater where the error was included in the research category, and the experimental method was further optimized. The subtraction method was used to filter out the interference of the background signal. At the same time, the repeatability of the cuvette empty cup with respect to air and the cuvette with respect to air were verified. The difference in absorbance, and the error caused by the difference between cuvettes was filtered by the method of subtraction, and a certain characteristic wavelength was selected to observe its absorbance data relative to pure water, which verified the use of optimized experimental detection. The method finally achieves the purpose of measuring the absorbance of seawater with high accuracy (＜0.000 5 AU). It was of considerable importance for detection of various substances in highly transparent water.