To understand the movement characteristics of a sudden point-source pollution in thermal-stratified reservoirs, the vertical movement of the thermal in the static thermal-stratified water was studied by experiment. The results show that the motion of the thermal is significantly different in water bodies with different temperature structures. The horizontal diffusion of the thermal is damped in linear stratified water. If the thermocline exists in the stratified water, the thermal can be retarded at the thermocline. Due to the density difference between the thermal and ambient water, Rayleigh-Taylor instability or double-diffusion convection may occur, and a finger-shaped intrusion can emerge. The vertical velocity of the thermal is positively correlated with the buoyancy but is negatively correlated with the cluster forward position and the buoyancy frequency.