The invention discloses a valve state on-line monitoring system and a working method thereof. The invention relates to the field of industry process control and detection, and in particular to improvements of technology of valve state on-line monitoring and fault diagnosis. The valve state on-line monitoring system has exquisite structure, is easy to use and has high reliability, and can use valve position (i.e., valve caliber and opening amount of a valve gate) and valve pressure to calculate a theoretical torque so as to compare the theoretical torque and actual torque and finally provide accurate determination of the state of the valve. An electric apparatus is connected to the valve, and is intended for driving the valve to perform opening and closing movement. A torque sensor is connected between the electric apparatus and the valve, and is intended for acquiring the current torque of the valve. A pressure transmitter is connected to a compression testing machine, and is intended for measuring current working pressure of the valve, and converts the pressure to a current signal. According to the invention, the system as a whole is characterized by exquisite structure, easy usage and high reliability.