Based on the meteorological data measured at a small meteorological station set up in Tongji University from 2009 to 2011, an analysis was made of the effects clearness index and solar altitude on the relationship between solar radiance and illuminance. It is found that the effects were nonlinear on the whole interval, and there was a slight difference at different intervals. Therefore, the piecewise linear fitting method was employed. The clearness index and solar altitude angle were divided into 9 intervals, and the linear fitting relationship between solar radiation and solar illuminance was established correspondingly. The measured data show that the method is of a higher accuracy, which is suitable for areas which have similar latitude and weather condition as Shanghai.