The invention relates to a circuit breaker, a transmission case and a transmission mechanism of the transmission case. The circuit breaker comprises the transmission case, wherein the transmission case comprises a case body and the transmission mechanism; the transmission mechanism comprises an upper contact pull rod, a lower contact pull rod, an insulating pull rod, an upper connecting plate and a lower connecting plate; the insulating pull rod drives the upper contact pull rod and the lower contact pull rod to move up and down through left and right movement; the upper connecting plate and the lower connecting plate are hingedwith the insulating pull rod through a first articulating shaft; the upper connecting plate is hingedwith the upper contact pull rod through a second articulating shaft; the lower connecting plate is hingedwith the lower contact pull rod through a third articulating shaft; the transmission mechanism also comprises a fixed articulating shaft; and the fixed articulating shaft is hingedwith an upper middle plate and a lower middle plate. In the process that the insulating pull rod drives the upper contact pull rod and the lower contact pull rod to synchronously move, impact loads, on guide structures of the upper contact pull rod and the lower contact pull rod, vertical to the movement directions of the upper contact pull rod and the lower contact pull rod are shared by the fixed articulating shaft, the upper middle plate and the lower middle plate, so that the impact loads on the guide structures are relatively small; the guide structures are not easily damaged; and the transmission stability becomesbetter.