The invention relates to a frequency conversion type automatic tension device of an extension band type conveyor, comprising a roller, a roller frame, an abdominal ring, a fixing plate, a terminal subassembly, a bearing subassembly, a coupling sleeve, a drive shaft, a speed reducer, a brake, a coupling, a variable-frequency motor, a hydraulic mechanism and an electric cabinet. The frequency conversion type automatic tension device of an extension band type conveyor adopts the design that a retracting roller and a double-planet bevel gear are used for changing directions and speed, a unidirectional wet brake is used for braking, and frequency-conversion speed regulation and hydraulic power are combined, the tension device can be automatically controlled to change speed and directions, brake and wind wire cables, and the like, the electric cabinet is controlled by a microcomputer processor, and a sensor acquires functional information. The frequency conversion type automatic tension device of an extension band type conveyor has advanced design, compact structure, convenient operation and use, high strength and large force, is safe, stable and reliable, and can effectively control the roller to rotate, change the speed and the directions, reinforce the retracting ability and satisfy the tension requirement of the extension band type conveyor under the conditions of long conveying distance, large conveying quantity and high speed.