Most of the researches on the bidirectional reflection distribution function (BRDF) are focused on the model establishment and application. The geometric attenuation factor (GAF), which plays an important role in the model, is based on the existing model. However, there are some limitations in the existing model, such as turning point, inability to keep bounded, and imperfect consideration in practical application. Based on the detailed analysis of the random surface model, a modified integral GAF is put forward, and the simulation is conducted to verify that the model can eliminate the bad behavior of BRDF when the reflection angle is large and keep the bounds within the range specified by GAF. The data obtained by the proposed model fit the existing BRDF data better through simulation, and this model explains the backward reflection phenomenon effectively.