Abstract: A theory analysis of real meaning of the output graph showed in particle simulation code (PIC) is presented in this paper, the field power viewer command is taken into account. The complex function of field power density is educed, the relationship of field power density and real field power is analyzed, and the phenomena of different graph at different axis location shown in the PIC simulation is explained perfectly. The simulation result of output power on 8mm band second harmonic gyroklystron is analyzed deeply and the real meaning of the output graph and correct pick-up of output power are given out.