From March 1988 to March 2001, 58 absolute gravimetric station determinations have been carried out by National Institute of Metrology cooperating with China Seismological Bureau with the NIM- II Absolute Gravimeter at Xiangshan seismostation among which 44 groundwater level observations have been carried out simultaneously. This paper studies the mechanism of gravity change at Xiangshan seismostation from various point of view. The main conclusions include: (1) the groundwater activity is the main local interferential source, the groundwater levels correlate with gravity observation values by segments and its effect on gravity can be corrected by a 5th polynomial; (2) the effect of local crustal deformation is very small, thus it can be ignored; (3) the earthquake activity can result in short-term change of the gravity field and its maximum magnitude comes to 0.333μ ms^-2;  (4) the gravity value approximately drops linearly by 0.191μ ms^-2 from 1988 to 2001, the average decrease is 0.0147μ ms^-2 per year, and this gravity change belongs to global or regional gravity change.