Polycarbonate multi-wall sheet(PC sheet) is widely used as greenhouse glazing material because its light weight, high insulation, strong resistance of impact and long service life. As its light transmissivity performance is not good enough, a lot of planters suffered damage in plant production, and some disputes related to the material often happened in China. In order to understand and analyze the light transmittance of this material, so as for greenhouse designers or greenhouse builders to choose and use it correctly, PC sheet light transmissivity was tested and calculated. As comparison to glass, which is mostly used in greenhouse because of good light transmittance, UV, visible and infrared light transmissivities of both glass and PC sheet were measured. The results show that PC sheet cannot transmit UV rays, and the visible light transmissivity in PC sheet is about 10% lower than that in glass, the infrared light transmissivity in PC sheet is roughly 40% of that in glass in 2～5 μm wave range, but there are two peak values of light transmissivity lower than 5% in PC sheet in 5～25 μm wave range, in which range light cannot transmit in glass. PC sheet is not as good as glass for greenhouse glazing material in light transmittance, but it is excellent in insulation.