The invention relates to a building deformation monitoring method based on InSAR. The method includes the following steps that S1, InSAR original data is screened; S2, PS points are extracted and screened in a monitored region; S3, geometric phase positions of the PS points are removed; S4, the space deformation quantity of a building is solved; S5, according to the solved deformation quantity, the gradient of the building is calculated; S6, monitoring is carried out on time dimension, the steps are repeated, a monitoring differential value is obtained, and the deformation quantity and deformation rate of the building are obtained. By the utilization of the InSAR technology, the deformation field of the building in the direction of a radar sight line is obtained, spatial interpolation is carried out on pixels in the deformation field in the direction of the radar sight line, wherein the coherence of the pixels is lower than an untwisting threshold value, and therefore a spatial continuous deformation field is obtained; by processing and calculating obtained data, the deformation and deformation rate of the building are obtained. The method is large in range and low in cost and easily monitors the building for a long term during monitoring of the building, and can effectively make early warning for accelerated deformation of the building.