The invention relates to a mortise and tenon type reverse cylinder lock core. Key holes are formed in the bottom of a front lock cylinder cover, grooves a are machined in the inner side wall, grooves b are formed in the bottoms of the grooves a, a key is arranged in the key holes, and cylinders are installed in the grooves a; a front inner lock core pad is fixedly installed in the center of the bottom in the front lock cylinder cover and in a cylindrical geometrical shape, through grooves e are formed in the cylindrical outer surface, an inner lock core body is placed on the front lock core pad, a lock core shaft is machined in the center of a disc of the bottom of the lock core body, the tail end of the lock core shaft is machined into a circular cylinder with the diameter smaller than that of the lock core shaft, grooves are machined in the circular cylinder, through grooves f are machined in the side face of the disc at the bottom of the inner lock core body, and a rear inner lock core pad and a rear lock core cover are sequentially installed on the lock core shaft of the inner lock core body; the rear inner lock core pad is fixedly installed on the rear lock core cover, through grooves g are machined inn the side face of the rear inner lock core pad, grooves c are formed in the inner side wall of the rear lock core cover, and grooves d are machined in the bottoms of the grooves c; spring grooves are formed by the grooves b and the grooves d and provided with springs, and cylinder grooves are formed by the grooves a and the grooves c.