A theoretical study on the underwater gas jet produced by moving vents is carried out. Due to the existence of moving velocity，the traditional jet model based on self-similar hypothesis is not suitable. In this paper， based on the characteristics of two-phase free turbulence generated in the water-air mixing process，and utilizing the vortex diffusion theory and the concept of Plant mixing length，a new mechanical mathematical model of axisymmetric underwater coherent jet is established，and the analytical solution of the coherent jet diffusion process is given. The jet boundary calculated by the analytical solution is in good agreement with the experimental results.
