The invention provides a method and a system for estimating ground PM2.5 based on a space-time regression Kriging model. The method comprises the steps of re-sampling ground PM2.5 observation data of a to-be-estimated region to a created mesh, and performing matching, wherein the matching process comprises the steps of averaging the ground PM2.5 observation data monitored in the same day by all PM2.5 stations in a mesh unit corresponding to the to-be-estimated region in the created mesh, and then assigning the averaged data to the corresponding mesh unit; calculating an experimental variance function of a residual error according to the ground PM2.5 observation data of the matched to-be-estimated region, and determining a space-time variance function model according to the experimental variance function of the residual error; performing fitting on the space-time variance function model by adopting a least square method; and estimating a ground PM2.5 concentration value of the to-be-estimated region by adopting the space-time regression Kriging model according to a fitting result of the space-time variance function model. Through the method and the system, the PM2.5 estimation precision can be improved.