Abstract: By analyzing lumped circuit of loaded resonant cavity, a new method of calculating external Q was advanced. In this way the resonant frequency and external Q can be determined with the reflected phase calculated in frequency domain. The particular calculations of reflected phase method, eigenmode method and time domain tracking method were shown in the software CST Microwave Studio. The three methods mentioned above were used to calculate the external Q of secondary emission microwave electron gun. Comparing the calculation results with measured data, it is indicated that these three methods are the same in high precision, but the calculating speed of reflected phase method is the quickest, whose calculating time is 1 071 s. At last, it is found that external Q is in inverse proportion to