The characteristics and effects of photosynthetic active radiation (PAR) on crops were analyzed on the basis of the observed radiation data from greenhouse. The results showed that a linear relationship between PAR and water surface evaporation exists, with the correlation coefficient of 0.7974. The photosynthesis rate (Pn) increases with the PAR increasing, but when the PAR reached a certain number, the photosynthesis rate stopped increasing. The cell sap concentration of leaf (CSC) shows a single peak curve with the PAR, but has a certain delay phenomenon relative to the PAR. The daily variation trends of the total solar radiation (Q) and PAR presents a single peak curve in sunny day or overcast day during winter or spring, Furthermore, the values in sunny day are higher than in overcast day. The utilization ratio of PAR for mini-watermelon in the autumn of 2007 and the spring of 2008 are 6.38% and 6.31% respectively, above the result of field crop.