For the requirement to control the content of traces SiO4-2 in the water and vapor in coal-fired power plants. By thie method of using spectrophotometry, the multichannel online monitoring SiO4-2 instrument is designed based on embedded system according to the Lambert-Beer theorem. The entire system is managed by the embedded real-time operating system uC/OS, based on the industrial grade embedded microprocessor AT9140800. The system satisfactorily realized the detecting of traces SiO4-2 in high purity water by the plunger pumps, cold light source monochromatic double paths optical system, “reversal reagent dosing” method. The experiments and field application show that the system has stably function, high intelligence grade, high test precision and good reproducibility, so it really meets the requirements of detecting traces SiO4-2 in different industries.