The embodiment of the invention discloses a bottom boundary determining method and device for a low velocity region. The method comprises the following steps: carrying out mesh generation on a near surface velocity model of a work region in the depth direction, and obtaining a mesh velocity model; determining at least one control point of the work region; aiming at each control point in the at least one control point, determining multiple meshes, which correspond to the control point, in the mesh velocity model; determining a depth value of a bottom boundary of the low velocity region of the control point according to a velocity ratio of every two adjacent meshes in the multiple meshes; determining the bottom boundary of the low velocity region of the work region according to the depth value of the bottom boundary of the low velocity region of each control point in at least one control point. According to the method and the device which are disclosed by the invention, the position of the bottom boundary of the low velocity region can be accurately determined.