Abstract: The insulation state of cable intermediate joint directly affects the stable operation of power grid cable system. In order to timely find out the change of insulation state of cable intermediate joint, we put forward a method of insulation state detection of cable intermediate joint based on the transmission characteristics of transient overvoltage signal in cables. Firstly, the distribution parameter model of the cable is established, and the expression of the voltage transfer function of the cable is deduced. The correlation and variation rule between the intermediate joint of the cable with two kinds of insulation defects and its voltage transfer function are revealed. Through the establishment of simulation calculation model, the voltage transfer function curves of the cable under various transient voltages are obtained. In order to realize the effective diagnosis of the insulation state of the intermediate joint of the cable, the quantitative calculation method of integrating the voltage transfer function curves is proposed. On this basis, the impact voltage test of 10 kV power cable is carried out to verify the reliability of theoretical analysis and simulation results. The results show that the change of voltage transfer function curve integral area S can accurately reflect the change degree of insulation state of cable intermediate joint and effectively identify the insulation fault of cable intermediate joint in the insulation resistance range of 100 Ω~10 MΩ, which provides a basis for the diagnosis of insulation state of cable intermediate joint.