The invention relates to an automatic tension control system and a method thereof, relating to a system and a method for applying pre-stress on a pre-stressed component in civil engineering. The system comprises an electric oil pump, a hydraulic jack, a controller of the automatic tension control system, a data relay controller and a computer. A pull rod type displacement sensor and a pressure sensor are arranged on the hydraulic jack. Control valve rods of left and right control valves of the electric oil pump are connected with a stepping motor. The controller of the automatic tension control system comprises a MCU (Micro-programmed Control Unit) single chip microcomputer, functional keystrokes, a displacement collection unit, a pressure collection unit, a buzzing and alarming unit, a display unit, a driving module, a motor steering output unit, a motor steering and stopping output unit and a motor pulse output unit. The method comprises the steps of tensioning, unloading and the like. According to the system and the method disclosed by the invention, the tension precision can be increased; the tension uncontrolled accident of the pre-stressed reinforcing steel can be effectively avoided; the system and the method have the characteristics of being simple in structure, low in cost, safe, reliable, low in quality monitoring difficulty, simple in method and convenient for operation; and the system and the method are applied to various fields.