Based on the repeated gravity observation data from 1996 to 2007 from the Longmenshan gravity network，which has been dealt with by adjustment processing，the benchmark interference removal and impact of elevation changes removal，and by using the 3-D inversion method to reflect underground density，we analyze the characteristics of Longmenshan regional dynamic crustal density at depths of 25km，20km and 15km． The results show that in the Wenchuan earthquake preparation process，the regional density field showed marked characteristics both in time and space distribution． From the point of time process，the density change trend in the ten years before the earthquake presents a periodic change pattern: steady phase，dramatic stage，slow reducing phase and slow increase phase． The degree of density changes is from large to small，which means that earthquake gestation has reached the final stage． From the point of space distribution， density change distribution has a tendency of “dispersion—relative concentration”，this shows that before the earthquake，the entropy of the underground density field was decreased． In addition，dramatic density changes often occur in the Longmenshan fault zone and western Sichuan plateau． Also，with the increase of depth，the trend of density change is more and more obvious． Through comparative analysis，the influence of density change on gravity is much bigger than that from height change．