Abstract: The Gas Electron Multiplier (GEM) has attracted wide attention due to its better position resolution and isotropic two-dimensional structure, GEM is also planned to be used as the TPC readout detector in CSR External-target Experiment (CEE) under construction. The transmission characteristics of GEM under different electric field conditions have great influence on the effective gain and energy resolution of the detector. In this paper the effects of the electric field in the drift region and the induction region on the transmission characteristics of the detector are performed on single GEM, and the effects of voltage distribution and electric field in induction region on the transmission characteristics of double-layer GEM detector were studied. The results show that in single and multi-layer GEM detectors, electric fields in the different region affect the transmission characteristics of the detector mainly by changing the electron transmittance and changing the avalanche field intensity and distribution of GEM, which will affect the effective gain and energy resolution of the detector finally. The above experimental results indicate that GEM detector is a good candidate for CEE-TPC readout detector and they also provide a reference for the selection of the working point of multi-layer GEM in TPC.