The invention discloses a cam control synchronous conveyer belt releasing mechanism for a three-knife trimmer. The cam control synchronous conveyer belt releasing mechanism comprises a cam, a rocker, a connection rod, a swinging block, a square shaft, a shaft sleeve, a synchronous belt wheel, a small shaft, a swinging arm and a pressure spring, wherein a roller is arranged on the cam; the cam is connected with the rocker through the roller; the two ends of the connection rod are separately provided with a joint bearing; the connection rod is separately connected with the rocker and the swinging block through the joint bearings on the two ends; the swinging block is connected with the square shaft; the square shaft penetrates through a square hole in the shaft sleeve to be connected with the shaft sleeve; the synchronous belt wheel is connected with the swinging arm through the small shaft; the swinging arm clasps the shaft sleeve; and the pressure spring is connected with the swinging arm. By adopting the cam control synchronous conveyer belt releasing mechanism, a synchronous belt can be released; and sections enter the three-knife trimmer and are aligned for trimming by lay gauges, so that the trimming precision is improved.