The invention discloses a displacement multiplication mechanism-based vehicle-mounted soil collecting device. When the soil collecting device is used for collecting soil, the work efficiency can be improved, the soil collecting precision is ensured, and the oil collecting depth can be increased. The soil collecting device comprises a power bearing vehicle, a support cylinder, a guide support frame, a hydraulic motor, a sampling pipe and a hoist cylinder, wherein the support cylinder is hinged to the power bearing vehicle; the front end of a piston rod of the support cylinder is hinged to the guide support frame which is hinged to the power bearing vehicle; the hydraulic motor is connected to the guide support frame in a sliding way through a sliding block; the sampling pipe is detachably connected to the output shaft of the hydraulic motor; the guide support frame has an enclosed box structure; the hoist cylinder is arranged in an inner space of the guide support frame and is connected to the bottom or the top of the guide support frame; a piston rod of the hoist cylinder is connected with the hydraulic motor through a displacement multiplication mechanism; and the displacement multiplication mechanism is arranged outside the guide support frame.