Using the four-period mobile gravity observation data from 2016 to 2019 in Northern Tianshan mountain we analyze the dynamic characteristics of one year scale gravity field,and defined the gravitational anomalies at different depths with the wavelet decomposition method. Utilizing the power spectrum analysis we obtained the depth of field source corresponding to each order of wavelet gravity details. The results show that there existed negative gravity changes with high value around the epicenter before Jinghe M_S6.6 earthquake on August 9,2017,in which the 4th order wavelet details showed an obvious four-quadrant distribution near the epicenter. Moreover,the epicenter of Kuqa M_S5.6 earthquake on January 16,2020 was also located in negative gravity area,and the values of wavelet details were relatively small. The depth of the field source estimated by the power spectrum is close to the focal depth of two earthquakes.