The invention discloses a size variable true triaxial test system which comprises first loading frames and a second loading frame, wherein each first loading frame comprises an upper end surface used for loading in the Z direction, a lower end surface opposite to the upper end surface in position and used for placing test pieces, a right end surface used for loading in the X direction, a left end surface opposite to the right end surface in position, and a rear end surface used for loading in the Y direction; loading devices used for loading the test pieces are fixed on the upper end surface, the right end surface and/or the left end surface and the rear end surface; mounting strips are arranged on the first loading frame on the outer side on the front end surface of the first loading frame on the relative position of the rear end surface; sliding grooves matched with the mounting strips in shape and size are formed in the second loading frame; a push-and-pull mechanism used for pushing the second loading frame to slide along the mounting strips is fixed on one side of the first loading frame on the outer side of the front end surface. The size variable true triaxial test system solves the problem of the limitation of test pieces in size in the conventional true triaxial tester.