A numerical investigation on the evolution mechanism and characteristics of the submerged laminar round jet in a viscous homogenous fluid is conducted by using the computational fluid dynamics method based on the incompressible Navier-Stokes equation. Three non-dimensional parameters for the mushroom-like vortex structure, including the length of the jet L*, the radius of the mushroom-like vortex R* and the length of vortex circulation d*, are introduced and the variation characteristics of these parameters with the non-dimensional time t* are quantitatively analyzed. Results show that there exist three distinct stages in the formation and evolution procedures of the mushroom-like vortex structure, including the starting, developing and decaying stages. In the starting stage, L* and d* increase linearly with t*, while R* approximately remains to be a constant; in the developing stage, a considerable self-similarity is confirmed, and L*, R*, d* display the same proportional relationship to t*1/2 regardless of the variations of Reynolds number and injection duration; in the decaying stage, L* and R*are approximately proportional to t*1/5, while d* nearly levels off as a constant. Moreover, velocity characteristics at the secondary backflow point and the momentum and geometry centers, the distribution features of the vertical vorticity, as well as the vorticity-stream function relationship are analyzed for the mushroom-like vortex structure.