In order to meet the needs of rapid and accurate detection of in-situ water total alkalinity, a mini online rapid tester of total alkalinity of trace in-situ water was designed by combining sequential injection analysis (SIA) and continuous spectrum detection. The system mainly applies the SIA and continuous spectrum detection method to the total alkalinity automatic titration detection process by designing the titration cell and conducting experimental research on a new method of determining the titration’s critical value by the continuous spectrum detection method. Based on the national standard of the determination on total alkalinity in industrial circulating cooling water and surface water, the detection process of in-situ water quality alkalinity was designed. Also, on the basis of the sequential injection technology as the standard of controlled titration process, under the conditions of continuous spectral scanning measurement of the solution detection process, phenolphthalein and methyl orange were used as indicators to perform a titration analysis on the total alkalinity of water quality. The phenolphthalein alkalinity and methyl orange alkalinity titration process were monitored by continuous spectral scanning. The peak value at 552 nm of the absorbance curve was used as the phenolphthalein titration threshold value. The absorbance curve’s peak value shifting from 465 to 504 nm was selected as judging conditions for the critical value of basic orange titration. The optimal indicator dosages were analyzed by the absorbance curves of the solutions obtained by adding different doses of phenolphthalein and methyl orange indicators, which were 0.01 and 0.04 mL, respectively. The system applied least-squares fitting algorithm to establish a regression model for total alkalinity determination and optimized the detection system and process. The experimental results indicate that the total alkalinity of water quality is linearly related to hydrochloric acid consumption in the range of 0.20 to 25.00 mmol·L-1 and the fitting coefficient of the working curve is no less than 0.994 2. The relative standard deviation (RSD) of the total alkalinity measurement’s repeatability is between 0.207 and 1.151%. Waste liquid volume is no larger than 16 mL. The lowest detection limit is 0.03 mmol·L-1. The recovery of spiked samples is between 97.2% and 102.3%. Also, there is no significant difference between the experimental results and the national standard method. The new method for determining the critical value of titration using continuous spectrum detection method is of great significance to improve the technical performance of the total alkalinity detector of water quality. It can be applied to many kinds of monitoring application platforms such as system grid monitoring of surface water, circulating cooling water, and breeding circulating water.