In order to maintain the stability of heating, the effect of frequency shifts range on heating pattern of domestic microwave was invested based on the numerical simulation method. In this study, the frequency of a microwave oven was measured under no-load and load conditions and a computational simulation model was developed to investigate the effect of frequency shifts on heating pattern of microwave heating. The results showed that the frequency of microwave was constantly changing during microwave heating in the oven; when the frequency shifts was large, the temperature distribution was not stable even heating the same food; the heating pattern is more stable as the frequency shifts reduced; when the range of frequency is limited to the centre frequency ±2 MHz, the heating pattern of food heated in microwave oven would be almost consistent.