Abstract: Thermionic RF gun-based electron accelerators can produce ultrashort electron beams. The alpha magnet is used for bunch compression. The transmission process of electron bunch in the alpha magnet is complicated. The research results described in the related literature were based on the ideal model, and the study of beam transmission in an alpha magnet which have an entrance hole in the mirror plate was important for the use of alpha magnet. The design model of alpha magnet is presented, which is the important part of high energy electron radiography facility. The beam transport properties between the ideal model and the alpha magnet are compared by beam dynamics simulation. It is verified that the designed alpha magnet model can effectively compress the electron bunch length. The beam dynamics under the optimization injection angle and space charge effects was discussed. It was found that the effects of the mirror plate entrance hole can be compensated by reducing the injection angle. The multi-bunch simulation results were also reported. The beam emittances and bunch sizes can be optimized by installing some quadrupole magnets between the thermionic RF gun and the alpha magnet.