Based on the analytical solution of RSJ model with the existence of external radiation
a method for finding voltage response and RF impedance of superconducting weak links is proposed and discussed. Taking the lowest approach
the analytical expressions of the voltage response and RF impedance are obtained with this method. The results show that the impedance varies continuously on the IVC from the 0th current step to the 1st step. The analytical impedance expressions within the range of the 1st current step are deduced for the first time.