Industrial boilers have strict requirements for water quality. It is necessary to adjust water treatment measures and regular blowdown according to the monitoring of phosphate and soluble silica content in boiler water to avoid boiler scaling, corrosion, and deterioration of steam quality, and prevent major accidents. The traditional boiler water quality detection technology has a strong lag and long detection time, which cannot meet real-time online detection. Aiming at the requirement of detection of phosphate radical and soluble silica content in boiler water quality, a portable and low-cost online detection system of boiler water quality based on STM32 was studied. The pump valve flow path is constructed with a syringe pump equipped with a syringe and a distribution valve, LED and silicon photocell are used for the light source and detector of the system respectively, and the photoelectric detection part is composed of a designed Z-shaped flow cell, which reduces the experimental cost; Design a data acquisition circuit based on STM32 microcontroller and a 16-bit high-precision analogy-to-digital converter, as well as a constant-current drive circuit for driving LED light sources, to improve the stability and high precision of the automatic measuring instrument. The system test results show that the boiler water quality online detection system based on STM32 can accurately detect the phosphate and soluble silica concentrations in boiler water quality.