Abstract: A comprehensive analysis is made of the characteristics of the traction current, pickup current and fault current of subways. It is shown that the protection performance caused by differential current increment is mainly determined by the separation effect of current increment, the current change rate di/dtand the checkout precision of every current increment. Since common detection and separation ways have poor anti-interference and unacceptable checkout precision, wavelet transform is used to detect the surge current. Then based on this, the every current increment is extracted out to solve the above problems. Simulation experiments prove that applying wavelet transform to current detection is more practicable and effective than other detecting ways.