The invention discloses a pipe expansion automatic detection and control device of a round pipe belt type conveyer. The pipe expansion automatic detection and control device of the round pipe belt type conveyer comprises a machine frame and a material input controlling mechanism. An unloading device, a pressing device and a pipe expansion detection rod are arranged on the machine frame from left to right in sequence. The material input controlling mechanism is connected with the pressing device, the pipe expansion detection rod and the unloading device. The pipe expansion automatic detection and control device of the round pipe belt type conveyer is provided with the material input controlling mechanism, the unloading device, the pressing device and the pipe expansion detection rod. The pressing device flattens materials. The pipe expansion detection rod detects the pressure value of the materials to the pipe expansion detection rod and transmits a pressure signal to the material inputcontrolling mechanism. The material input controlling mechanism converts the received pressure signal into a filling rate. When the actually measured filling rate is larger than or equal to a critical filling rate, the material input controlling mechanism controls feeding of a hopper and meanwhile controls the unloading device to work, surplus materials are unloaded, and a pipe expansion accidentis effectively avoided.