Abstract: Based on ray tracing method, the laser transmission model of typical tapered multimode fiber was established. Integrating the peculiarity of jamming laser beam from the laser jammers, the coupling efficiency and transmission modes of laser spot throughout the tapered multimode fiber were calculated. Experiments were designed and conducted to verify the simulation. The results indicate that the output laser spot behaves circular when a laser spot with Gauss distribution incidences the tapered multimode fiber; the coupling efficiency decreases when the incident angle increases, and the decrement rate decreases as the incident angle increases. In order to solve the circular laser spot distribution, a new method of a small angle bending of the connected cylindrical fiber was introduced, and the output laser spot was improved to two-dimensional normal distribution, the theoretical coupling efficiency was about 80%. This provides a theoretical and experimental support for jamming laser intervening into the laser guided weapon countermeasure hardware-in-loop system.