In the process of Mask Projection Stereolithography (MPS), the value of illumination on the view surface of MPS has serious influence on building accuracy. In order to improve the building accuracy of MPS, it is necessary to measure the illumination accurately. An illumination measuring system with Silicon Photocell is designed and developed. The system uses photocell to convert light signal into weak current signal, which is then sent to micro control unit in the form of voltage signal to carry out A/D conversion after I/V switching and amplification, and then the system calculates the actual illumination value according to the digital value and the linear sensitivity of Silicon Photocell. For adjusting gain of the amplification circuit automatically, the system adopts digital potentiometer as feedback element. The illumination value on the view surface of MPS is measured by the system, measured data is fit with MATLAB, and the relationship between illumination and gray is established in order to adjust illumination distributing on the view surface of MPS by changing gray.