The invention relates to a method for automatically testing line differential protection based on an RTDS. The method includes the steps that line differential protection test programs are set by an RTDS script setting module; a line differential protection model is established and operated through a protection model operating module; the line differential protection test programs are operated sequentially through a script operating module in the line differential protection model; test results are stored as waveform files according to corresponding test program names; the waveform files are read through an MATLAB module and experiment results are generated according to the waveform files; a test report is output through a test report output module according to the experiment results. Compared with the prior art, the method has the advantages of being short in test time, high in accuracy and the like.