Glaciers are main factors affecting climate change. SAR (synthetic aperture radar) not only has the characteristics of high spatial resolution and multi-polarization, but also is sensitive to changes in dielectric constant caused by ice and snow melting. It has become an important data source for monitoring the ablation changes of mountain glacier surfaces. Based on the Sentinel-1 time series data, this paper proposes a glacier surface ablation monitoring method based on multi-temporal and multipolar SAR change detection algorithm and Sigmoid function. Taking Laohugou No. 12 glacier in Qilian Mountain as the research area, 64 transects in the east and west branches of the glacier obtained by the "centerline-circle" method were firstly used, and 7 typical transects and the glacial retreat periods in 2019 and 2020 were determined by the SAR backscatter coefficient change characteristics of 124 time phases from 2019 to 2020. Then, based on the comparison of the reference image of the same track with the multi-temporal snowmelt image, the VV (vertical vertical) and VH (vertical horizontal) polarization backscattering coefficients of the change monitoring images were obtained, respectively, and the wet snow pixels were extracted after the weight combination was combined. Finally, the Sigmoid function was used to extract dry snow combined with the average altitude of wet snow, so as to obtain the distribution of dry and wet snow on the glacier surface in 16 time phases during the two-year glacial melting period. The accuracy of the extraction results was verified by using the UAV (Unmanned Aerial Vehicle) DOM (digital orthophoto map) on September 1, 2019 and the five Sentinel-2 images of the adjacent date of Sentinel-1, and the error results were analyzed in depth based on the temperature and precipitation data of the meteorological site on the glacier. Experimental results show that this method can effectively extract the distribution of dry and wet snow on the glacier surface, and the overall classification accuracy OA (overall accuracy) is as high as 96%, and the kappa coefficient is as high as 0.84. There are interannual variations in the melting period and melting characteristics in Laohugou No. 12 glacier. The melting period of glaciers in 2019 was from early May to mid-to-late September, and the melting period in 2020 was from mid-to-late June to early September. Starting from the melting season in 2019, the dry snow area on the glacier surface decreased rapidly, while the wet snow and glacial ice area were large during the entire melting period, but with the emergence of heavy snowfall, the dry snow area increased sharply. In 2020, the glacial ice and wet snow area changed significantly, the glacier ice area continued to increase, the wet snow area continued to decrease, and the dry snow area tended to be stable at high altitude.