The invention discloses an on-site large-scale direct shear test device for detecting the shear strength of cohesive soil. The on-site large-scale direct shear test device comprises a sample box, a lateral force test jack and a lateral displacement dial indicator, and further comprises a force bearing framework base. The sample box is used for containing cohesive soil samples and controlling stress on the samples. The lateral force test jack is connected with a lateral hydraulic pump through a lateral pressure gauge. The lateral displacement dial indicator is anchored through a fixing support. The force bearing framework base is anchored through a ground anchor and located on the outer side of the sample box. The inner side wall of the force bearing framework base and the outer side wall of the sample box abut against the two ends of the lateral force test jack respectively. The lateral displacement dial indicator is connected with the outer side wall of the sample box. The detection direction of the lateral displacement dial indicator is parallel to the arrangement direction of the lateral force test jack. The on-site large-scale direct shear test device has the advantages that the structure is simple, installation is easy, data of the shear strength of the cohesive soil can be obtained truly and effectively, and the on-site direct shear test device is suitable for various places and can be widely applied to the technical field of cohesive soil detection.