Time-lapse electrical resistivity tomography (ERT) has been proved to be a useful method for monitoring subsurface changes. Four-dimensional (4D) inversion scheme is one of the most common inversion schemes used to obtain subsurface resistivity at different moments. However, it uses the cross-time term to regulate the models at neighbouring moments and still needs to be improved. Therefore, we proposed a full-gradient difference (FGD) inversion scheme based on the 4D inversion scheme. The numerical experiment shows that the FGD inversion scheme has good imaging and anti-noise capability, which can be used in ERT monitoring.