An L-band high-power relativistic backward wave oscillator is designed. In the simulation, microwaves centered at 1.6 GHz are generated, with the power of 3.6 GW and the efficiency of 40%. In the preliminary experiment, the pulse duration of the device was only 45 ns, presenting a pulse-shortening phenomenon. Through the 3D particle-in-cell simulation analysis, it was found that the accelerating diode resonances significantly impact the operation of the L-band high power relativistic backward wave oscillator, and the resonance of a TE11 mode in the accelerating diode played the primary role in the pulse shortening. Moreover, we found that choosing the appropriate distance between the cathode baffle and the end of the annular cathode is beneficial to effectively suppress the starting oscillation of the parasitic TE11 mode. In the improved experiment, we changed the distance between the cathode baffle and the end of the annular cathode from previous 5.4 to 4.6 cm. Eventually, when the diode voltage is 650 kV and the diode current is 14 kA, microwaves centered at 1.58 GHz are generated with the power of 3.3 GW, the efficiency of 36%, and the pulse duration above 104 ns.