The invention provides a device preventing blocking and tangling of a sewage pump in the process of conveying of media including sewage and the like. A movable and static debris cutter coaxial with animpeller is arranged in front of an impeller water inlet, a pump shaft drives the impeller to rotate, meanwhile, a debris cutter movable cutter is also driven to rotate, debris in the conveyed mediais cut and crushed, and the phenomena of blocking and tangling caused by overlarge grains or overlong fiber are prevented. A motor automatic alternating positive and negative rotation control method is put forward, a current monitor is used for monitoring the motor current, accordingly the situation of impeller blocking or fiber debris tangling is recognized, and the motor is stopped and rotates negatively automatically. By means of double interlocking circuits inside contactors, the motor main control loop short circuit phenomenon occurring during conversion of the positive and negative rotation and the operation period of a motor control loop is effectively prevented. According to the device, the situation that blocking and tangling cause motor overload operating and burning out of the motor can be effectively prevented, and the whole device is high in efficiency, simple in structure and convenient to operate.