High-voltage fast transform switch is applied to improve the reliability of the power supply and power quality. However, most of the existing fault detection technologies need several milliseconds to detect short-circuit fault. It’s a bit longer for fast transform switch. In this paper, wavelet transform is applied to high-voltage transform line on the early detection, being characterized with the fourth dimension detail component of wavelet transform. The method is verified on Matlab/Simulink platform. It can separate short-circuit fault from overvoltage and inrush current caused by no-load switching, load switching and reactive power compensation. The simulation result shows the wavelet transform can identify the fault within 0.2 ms. Combining wavelet transform early detection technology with high-voltage fast transform switch will contribute to a stable and reliable power system. This work is supported by National Natural Science Foundation of China (No. 51377023).