Abstract: Light intensity is one of the indispensable factors for plant growth and transpiration. Real-time monitoring light intensity and guiding irrigation by it can stimulate plant growth, simultaneously, and play a role in saving water and energy. In this paper, a practical light intensity detection system was designed in a simple way with low-cost. The silicon solar panel was used as the solar radiation sensor, which could directly transfer the sun’s radiant energy into electric signals. The PIC16F876A MCU was used as the processor. The least square procedure was used to establish the model between the output voltage of silicon solar panel and the light intensity, then experiment was carried out to verify the performance of this system. The result showed that the average relative errors were around 1.19% in sunny days, around 1.57% in cloudy days, around 7.19% in rainy days, around 6.15% in changing weather, respectively. The average relative error was always below 10% in different weathers. The system accuracy was increasing while the light intensity increased. The system worked more precise when the light intensity was above 15 000 lx with the average error of 1.41%. The resolution was 0.1 lx. The system repeatability error was very low (<0.63%), which means the system was running in high stability. In summary, this system could work stably in the solar greenhouse in different weathers.