Disclosed is an indirect transmission type automatic frequency modulation filter including a microwave body, a moving plate, a fastener, and a motor unit. The microwave body has plural openings. The moving plate includes a board, plural lugs and a first moving member, and the lugs and the first moving member form the board. The fastener includes a support base and a second moving member. The motor unit includes a rod and a driver, and the driver determines a clockwise rotation or a counterclockwise rotation and drives the rod to move the moving plate towards or away from the microwave body so as to insert the lugs into cavities to a desired depth and adjust a center frequency.