The invention discloses a loading device and loading method for shearing test pretightening force. The device comprises an active shearing pressure head for applying shear loads to a rock sample, and a driven loading pressure head, wherein the driven loading pressure head is staggered with the active shearing pressure head on the loading end surface for applying the pretightening force to the rock sample; the active shearing pressure head and the driven loading pressure head are crossways arranged at two sides of the sample; the driven loading pressure head comprises a loading segment and a loading secondary segment which protrudes out of the loading segment on the cross section; and the front end surface of the loading secondary segment corresponds to at least one part of the rear end surface of the active shearing pressure head in area to apply the pretightening force to the active shearing pressure head from the rear end.