The present invention provides a blind cutting size setting mechanism and a blind cutting size setting method of the same for a blind cutting machine. The blind cutting size setting mechanism includes a base frame, a motor fixedly mounted on the base frame, a locating frame, the locating frame having an open frame body fixedly fastened to the base frame and a transmission screw rod fastened pivotally with the open frame body and coupled to the motor and two guide rods fixedly fastened to the open frame body, and a positioning control unit adapted to move in the locating frame between a start position and an end position upon operation of the motor to stop the blind to be cut in position for cutting, the positioning control unit having a screw hole and two through hole respectively coupled to the transmission screw rod and guide rod of the locating frame for enabling the positioning control unit to be moved along the guide rods upon rotary motion of the transmission screw rod.