Abstract:This article is based on the 2009-2011 measured meteorological data, which is recorded by small meteorological station set up in tongji university, and the effects clearness index and solar altitude on the relationship between solar radiance and illuminance are analysed. It is found that these effects are nonlinear on the whole interval, and there is slightly different in different intervals. Therefore, this paper uses piecewise linear fitting method. The clearness index and solar altitude angle are divided into 9 intervals, and linear fitting relationships between solar radiation and solar illuminance are established correspondingly. The measured data indicate that it has a higher accuracy, and it is suitable for areas which have similar latitude and weather condition as Shanghai.