Large free-surface vortex with air-core easily occurs in front of gate at the side crest outlets. A series of physical model tests were conducted to observe the flow patterns, analyze the cause of vortex formation, study the influences of operating head, gate opening, distance from side pier to the reservoir bank and the operation conditions of crest outlet adjacent to side crest outlet on vortex, and discuss the effect of vortex on pressure of crest outlet surface. The results demonstrate that, the large vortex with air-core is mainly generated by flow around side pier, which is a special hydraulic phenomenon occurring at the side crest outlets. The air-core diameter of vortex decreases with the increase of gate opening, and increases with increasing operating head and distance from side pier to the reservoir bank; moreover, the vortex is larger when the gate of crest outlet adjacent to side crest outlet in fully opening than in partially opening conditions; the vortex leads to the occurrence of sub-pressure at the sidewall and even cavitation damage, so it should be controlled for practical engineering.