Abstract: The penetration,interception,reflectance and extinction coefficient of radiation of summer maize canopy are analyzed.The penetration,reflectance and extinction coefficient all show an obvious diurnal variation under clear sky;there are remarkable changes for interception and penetration before and after flowering.The daily reflectance decreases along with crop growing period,which is as high as 0.23 at the initial stage and equal to 0.14 at the last growing stage.Over the whole growing period,the average is about 0.19.The canopy extinction coefficients for incident short and net radiation are 0.51 and 0.41,respectively.A concept of bulk extinction coefficient is firstly introduced,which may be easily used to calculate the absorbed radiation by canopy.The variation of bulk extinction coefficient is more stable than that of the traditional extinction coefficient,on average,the bulk extinction coefficient is about 0.26.