A constant-pressure spillover system is designed to produce a submerged Laminar round jet with the constant outlet velocity, the flow patterns are visualized by dyed liquid, the evolution mechanism and characteristics due to the jet momentum in viscous uniform fluid is conducted experimentally for different combinations of the injection time and Reynolds number, where three non-dimensional parameters for the mushroom-like vortex structure generated by such a submerged laminar round jet, including the length of the jet L*, the radius of the mushroom-like vortex R* and the length of vortex circulation d*, are defined and the variation characteristics of these parameters with non-dimensional time t* are quantitatively analyzed. Serial experimental results show that the formation and evolution process of such a mushroom-like vortex can be described as three stages: starting, developing and decaying stages. In starting stage, L* and d* increase linearly with t*, while R* approximately remains a constant. In developing stage, the mushroom-like vortex structure shows a significant self-similarity, where L*, R* and d* are approximately proportional to t*1/2 for different combinations of the injection time and Reynolds number, the experimental results agree well with the theoretical solution based on stokes approximation. In decaying stage, two kinds of decay cases will happen for the mushroom-like vortex structure. In the first case, the decay happens after the end of the submerged round jet, where L* and R* are related to t*1/5, and d* approximately remains a constant. In the second case, the decay happens before the end of the submerged round jet, where some broken phenomenon will occur for the mushroom-like vortex structure after the jet momentum reaches the critical value.