Abstract: Pulse jamming is widely used in electromagnetic environment of modern battlefield. In order to improve pulse jamming application efficiency in navigation countermeasure, the theoretical derivation and numerical simulation about jamming effect evaluation indexes of discrete spectrum C/A code signal and continuous spectrum M code signal are performed respectively under the influence of pulse jamming parameters, which are based on the process of GPS receiver and evaluated by indexes of jamming efficient carrier-to-noise ratio and code tracking error. To ensure effect of jamming, for the C/A code signal, pulse jamming frequency offset can be preset after calculating and comparing by acquiring PRN number information of C/A code, pulse repetition period can be set to 1ms and pulse duration can take value between 0.01~0.05 ms, but for the M code signal, the jamming frequency offset can be preset after calculating and averaging according to the method of acquiring single frequency jamming frequency offset, the jamming effect can be ensured by decreasing pulse repetition interval and increasing pulse amplitude, when the pulse repetition interval is fixed, pulse duty cycle can take value between 0.002~0.005. The conclusion reveals the influence essence of pulse jamming to GPS signal, which is significant for improving pulse jamming application efficiency.