Aiming at the difficulty of predicting the length of rock-breaking arc plasma channel by high voltage pulse discharge, a comprehensive test platform for rock-breaking arc plasma channel by high voltage pulse discharge was constructed. The development characteristics and typical current and voltage parameters of arc plasma channel under granite-tap water combined medium were measured, and the broken region formed on the rock surface under different electrode spacing and pulse discharge times was extracted. Based on the energy balance equation, the impedance model of the arc plasma channel in rock is established. The approximate optimal solution of the impedance model parameters is obtained by iterative optimization algorithm. The relative error between the calculated results and the experimental results is less than 7%. Based on the optimized parameters, the length of plasma channel is predicted by the measured current and voltage data. The absolute error between the plasma channel length predicted by the model and the measured value is in the order of mm, and the relative error is less than 10%, which provides theoretical support for the matching design of power-electrode load in the high-voltage pulse discharge rock breaking system.