Monitoring snow distribution area plays an important role in researching climate change and energy exchange process. Chinese small satellite constellation (abbreviated HJ constellation) is special for environment and disaster continuously monitoring or forecasting. By using HJ-1B CCD and infrared data or only using its infrared data, it can construct NDSI (Normalized Difference Snow Index) or MNDSI (Modified Normalized Difference Snow Index) respectively to detect snow area. However, obtaining the CCD and infrared images at the same time is impossible for the long time monitoring. Furthermore, snow area is mixed with vegetation, the snow index's value will be reduced, and snow area in these places could not be detected accurately. Therefore, this paper introduces an improved snow cover monitoring method based on MNDSI and priori information of vegetation to increase the detection accuracy of snow area by using HJ-1B infrared image. The proposed method has the higher precision than the compared method.