The radiation response of three-dimensional structure of ground objects is different from the reflection characteristics of their surface material at the pixel scale, which makes it difficult to extract and recognize the objects′ imaging features. Combined with the field experiment of the zero sight distance measurment of three-dimensional (3D) geometry, the influence of geometric structure on the composite reflectance of image pixels is analyzed. Based on the linear mixed model, some correction factors such as directional reflection characteristics, light shadow and end-member distribution are introduced into the model to improve its function. The results show that compared with the widely used linear mixed model, the accuracy of the improved model is increased to a certain extent for calculating the composite reflectance of the mixed pixel, and the relative error is reduced from 13.08% to 8.79%. In view of the complexity of the synthetic radiation effects of the 3D heterostructure mixed pixels, the completeness and universality of the improved model need to be further studied and verified.