In order to deeply study the surface movement law during mining under a thick loose layer, taking the Heze mining area in Shandong Province as an example and based on the field measurements of surface deformation, 12 numerical models were established using FLAC3D at different thickness ratios of the loose layer to the bedrock (hereinafter referred to as the loose-to-base ratios) ranging from 0.25 to 5.00. The surface deformation parameters were obtained, analyses were carried out on the stress of the overlying rock and the characteristics of surface deformation, and studies were conducted on the variation law of the surface deformation parameters. The results are as follows: (1) The surface subsidence coefficient is relatively large and the surface subsidence response is rapid because of the combined influence of the thick loose layer and the aquifer subsidence caused by water loss. The starting distance is 0.2H (H represents the average depth). The recession characteristics of the boundary area of the subsidence basin are obvious, and expand rapidly first and then slow down until finally converging. (2) When the mining depth remains constant, as the loose-to-base ratio keeps rising, the subsidence coefficient initially goes up and then declines. The horizontal movement coefficient first increases, subsequently decreases, and ultimately becomes stable. The tangent of the main influence angle also shows an upward-then-downward trend. It is found that when the loose-to-base ratio ranges from 1.50 to 2.00, the surface deformation attains its peak value. (3) Above the center of the goaf, the failure of the overlying rock is mainly tensile failure. As the loose-to-base ratio increases, the tensile stress first increases and then decreases. Above the boundary of the goaf, it is affected by both tensile and compressive stresses. The compressive stresses on both sides of the goaf decrease as the loose-to-base ratio increases.