Aiming at the requirements for accurate and rapid combined determination of ammonia nitrogen and chemical oxygen demand in surface waters of class Ⅰ-Ⅴ, groundwater and industrial wastewater, this paper combines continuous spectrometry and Sequential Injection Analysis (SIA) in spectral Analysis, based on the national standard for surface water detection, taking in-situ water quality parameters of ammonia nitrogen (NH3-N) and chemical oxygen demand (COD) as the detection objects, designing a micro, efficient and rapid detection instrument for NH3-N and COD in-situ water. The system mainly relies on the self-designed digestion cell structure based on ultraviolet lamp digestion with heated closed digestion, and the detection cell structure based on the spectral scanning design to achieve the purpose of rapid digestion and stable detection. It also optimized the detection process based on spectrophotometry. At the beginning of COD digestion, the coordination compounds in the detection tank after NH3-N index coloritization were determined by spectral scanning. After digestion, COD was determined, the whole detection process was shortened by at least 60 minutes compared with the national standard detection method. It can automatically complete the determination of NH3-N and COD within 25 minute, greatly saving time cost. Plotting the absorbance and continuous wavelength curve of coordination compounds after spectral scanning color reaction: NH3-N and COD have obvious absorption peaks at 690 and 445 nm, respectively. After reading the absorbance value at the peak, the least square method is used to establish regression modeling for NH3-N and COD, fitting the regression equation and calculating the correlation coefficient, and drawing the absorbance-concentration working curve of the corresponding parameters. The experimental results show that the correlation coefficient r of the NH3-N standard working curve is≥0.998 7 in the concentration range of 0~2 mg·L-1, and the concentration is positively correlated with the absorbance. The relative standard deviations of repeatability were 1.36%～1.68%, and the recoveries were 97%~102%. In the range of 0~50 mg·L-1, the correlation coefficient r of COD standard working curve is ≥0.997 8, and the concentration is negatively correlated with the absorbance. The relative standard deviations of repeatability were 2.14%~2.48%, and the recoveries were 97.6%~102.95%. The system is accurate, linear and stable, and has high feasibility and reliability. Research on the method of combined determination of NH3-N and COD based on SIA and continuous spectroscopy is of great value in the research to broaden the application of spectroscopy in the field of rapid water quality testing and to improve the efficiency of detection.