The three-dimensional velocity field measurement system V3V was used to measure the flow field under the bell mouth of vertical axial-flow pump. The velocity gradient, vortex intensity and vortex kinetic energy in the vortex region during the evolution of floor-attached vortex were analyzed. The experimental results showed that the formation process of the floor-attached vortex was accompanied by the constant changes of velocity gradient, vortex intensity and vortex kinetic energy. The velocity gradient, vortex strength and kinetic energy of vortex in the vortex region was firstly increased with time and reached the maximum, and then maintained for 0.4s, and then decreased rapidly. The duration of the formation and development of the floor-attached vortex was longer than that of the collapse and disappearance of the vortex. The vortex intensity was changed in different stages and positions reflected the characteristics of vortices in the evolution of vortices. The intensity of the floor-attached vortex was firstly increased with time, then maintained for a period of time, and then decreased rapidly. In the initial stage of vortex development, the vortex intensity was gradually decreased with the increase of the height from the bottom of the pump sump, after the floor-attached vortex entered the maintenance stage, the floor-attached vortex satisfied the vortex intensity conservation law; affected by the complex environment in the bell, the vortex was firstly collapsed and disappeared from the impeller inlet, and the floor-attached vortex intensity was gradually decreased from up to down. The research provided theoretical guidance for the optimal design of the pump sump.