The experiments on ECRH trapped electron beam injection in various ways have been carried out on a simple mirror-MM-2 device by use of a 15GHz high power gyrotron. The experimental results show that the preionization time of plasma will be greatly reduced for the electron beam injection ahead of the microwave. And because of ECRH trapped electron beam, the parameters of hot electron ring will be improved greatly, and the diamagnetic value increases about 62 percent. The trapping efficiency of electron beam for the increasing rate of diamagnetic is about 30-40 percent. The gas "pressure window" for building a hot electron ring is (4.67-21.3)×10-4 Pa, as the gyrotron output power is about 30kW.