The invention relates to a pit-digging volume detection device and method for detecting the subgrade compaction degree of a pit-digging method. The pit-digging volume detection device comprises a piston cylinder, and a piston is assembled in the piston cylinder in a guide mode. A piston push rod is connected to one side of the piston, a solid flexible body for filling the corresponding to-be-drilled road surface or detection hole by being ejected and pushed by the piston is arranged at the other end of the piston. The solid flexible body is of a cylindrical structure with the outer diameter same as the inner diameter of the piston cylinder. The pit-digging volume detection device further comprises a stroke detection structure for measuring the movement stroke of the piston. The pit-diggingvolume detection device and method solve the technical problem of high work intensity caused by sand recovery due to the fact that sand filling is used for performing pit-digging volume detection inthe prior art.