In the particle acceleration by stimulated emission of radiation (PASER), the energy stored in an active medium transfers directly to the electron beam passing through in discrete amounts by emitting a photon when the bounded electron returns from upper to lower energy state. The wake-field generated by a bunch of electrons traversing in an active medium was discussed. The calculations of the development of amplitude for gas mixture active medium about CO2 and ArF were taken respectively. The results show that the gradient can reach around 1 GeV/m，which is about two orders of magnitude larger than that in normal acceleration structure (such as disk-loaded waveguide acceleration structure) in present. In addition, the electron energy gain occurring as a train of electron micro-bunches traversing in gas mixture was analyzed by a two-dimension model. The train of micro-bunches can obviously gain energy from the active medium and energy exchange is linearly proportional to the interaction length d. The influence of the bunch figure and other quantities on the energy exchange occurring as a train of electron micro-bunches traversing CO2 gas mixture was investigated when the interaction length is 0.5 m. The results illustrate that maximum electron energy gain can be obtained by the train of micro-bunches with optimized parameters.