A new high-gain over-moded microwave pulse compression system has been found. The power gain of the system, in the optimal resonant state, has reached up to 280.04:1.00 for the rectangular TE1,0,20 resonant mode at 2.920GHz in S-band. Relative the traditional physical model before, the power gain of the new physical model increased by 51.86%; It has an important potential application to generate an ultra-high power microwave pulse of 28GW and 13ns in experiment rather than in theory. What the advanced technoloy and theory, see this letter to the international top SCI magzine of ” IEEE Transactions on Microwave Theory and Techniques ” .