An automatic tensioning device for belt conveyor is suitable for long-range covneyor or a rubber belt conveyer, of which belt has a large extension rate. It achieves a large regulative range, an accurate control of tension force, and stable adjustment. The system consists of a tension monitor, a hydraulic device and a hydraulic hoist. The tension monitor comprises a detecting cylinder, a pressure sensor, a shutoff valve and a pressure meter. The hydraulic device comprises a pump, a reversing solenoid valve, a overflow valve, a manual valve, a hydraulic one-way valve, a proportional reversing valve, a one-way throttle valve, a one-way valve, a throttle valve, a tank, and oil-way. The outlet of the pump is respectively connected with the tension monitor, the cylinder and the driving circuit of the hydraulic hoist via the one-way valve.