Based on CHIPIC platform and the ionization physics mechanism between electron and gas, a full three-dimensional particle-in-cell/Monte Carlo (PIC/MCC) code is developed with the MCC ionized collision module. The simulations of a relativistic backward wave oscillator with helium gas are used to verify the code. The results show that the gas can enhance the current, peak power and pulse length, but too much gas leads to the reductions of peak power and pulse length.