High repetition frequency laser jamming is a highly effective jamming technology in the field of opto-electronic countermeasure. Starting from the principle of high repetition frequency laser, the decrease of the signal-to-noise ratio (SNR) of the receiving system in a laser seeker caused by jamming laser power is analyzed theoretically and calculated numerically, and hence the previous overall understanding of high repetition frequency laser jamming is clarified. The numerical calculation result shows that for a missile guidance system, its SNR may be reduced with the increase of jamming laser power at a given distance and may be reduced with the increase of the target distance in the case of same jamming laser power. When the target is at the distance of 6000 m and the jamming laser power is greater than 0.4 W, the SNR of the missile guidance system may be reduced to less than 1. At this time, the missile guidance system can not detect the target echo signals from different noises, and so the target to be attacked can be protected effectively.