A new method for multi-parameter water quality detection with variable optical range has been proposed, addressing scientific and technical challenges in the current national standard analytical methods. This method utilized ultrasound and micro-nano bubble (US-MNB), continuous spectrum analysis, and sequential injection analysis (SIA). A water quality multi-parameter detection system was designed and validated for the analysis of total phosphorus (TP), chemical oxygen demand (COD), ammonia nitrogen (NH3-N), and hexavalent chromium (Cr6+). The feasibility of the new method was verified. The systems core consists of a digestion chamber integrating ultrasound and micro-nano bubbles and a detection chamber with an adjustable optical range. This structural design enabled rapid digestion and stable detection. The multi-parameter detection process was optimized based on national water quality testing standards. Spectrophotometry and sequential injection analysis techniques were employed for continuous spectral detection of the four water quality parameters. Firstly, the TP was digested by a US-MNB with strong oxidants at normal temperature and pressure. At the same time, the complexes were directly determined by spectral scanning after the color reaction of NH3-N in the detection chamber, and thenthe TP was determined after digesting. Similarly, the compounds in the detection chamber after the chromogenic reaction of the Cr6+ were directly determined by spectral scanning. At the same time, the COD was digested, and then the COD was measured after digestion. The time used for the whole detection process was greatly reduced, and the determination of multi-parameter of water quality can be completed automatically in a short time, significantly improving the efficiency of detection. In this system, four water quality parameters(TP, COD, NH3-N, and Cr6+)were measured, combined with the least squares method to establish a regression model, fit the regression equation and calculate the correlation coefficient, and plot the concentration-absorbance standard working curve of each parameter. The results showed that the TP standard working curve had a fitting coefficient ≥0.984 5, with a positive correlation between concentration and absorbance. The repeatability (RSD) ranged from 3.05% to 3.62%, and the spike recovery rate was 97.8% to 103.6%. The COD standard working curve had a fitting coefficient ≥0.998 7, with a negative correlation between concentration and absorbance. The repeatability (RSD) ranged from 2.12% to 2.74%, and the spike recovery rate was 98.7% to 104.7%. The NH3-N standard working curve had a fitting coefficient ≥0.995 3, with a positive correlation between concentration and absorbance. The repeatability (RSD) ranged from 3.41% to 3.59%, and the spike recovery rate was 99.2% to 102.4%. The Cr6+ standard working curve had a fitting coefficient ≥0.993 8, with a positive correlation between concentration and absorbance. The repeatability (RSD) ranged from 3.51% to 3.92%, and the spike recovery rate was 98.9% to 109.3%. The system accurately determined the content of TP, COD, NH3-N, and Cr6+ in water samples and demonstrated excellent stability and reliability. The studyon the multi-parameter detection method with variable optical range in water quality based on US-MNB is of great significance in broadening the application of spectroscopy in the field of rapid detection of water quality multi-parameters and improving detection efficiency.