The classic astrometric instruments can measure the local astronomical longitude and latitude by using plumb line as a reference, where the variation of local plumb line can be gotten. The deflection of the local plumb line implies the change of the gravitational field around the station. It is often related to the redistribution of subsurface matter, and could be helpful for knowing the situation of underground materials. A dual-mass model is developed to find the relationship between local PLV (Plumb Line Variation) and subsurface mass change. 3480 sets of cases are considered which containing positive and negative mass variations (relative to the geological background) in the range of 97◦E–107◦E (E denotes east longitude) and 21◦N–29◦N (N denotes north latitude). The relevant problem is solved by the DE (Differential Evolutionary) algorithm. The resulting error of position is about 1 meter and error of mass is less than 1011 kg, which are improved in comparison to the previous treatments.