Heat transfer and aerodynamic performance in worn squealer tip gap of a high-pressure gas turbine stage were numerically investigated. Effects of the starting location of wear and wear depth on tip heat transfer coefficient distributions and stage efficiency were analyzed to evaluate the aero-thermal performance degradations in the gas turbine stage after wear. At three starting locations of wear and five wear depths, flow patterns in worn squealer tip gap of the turbine stage were visualized and compared with the original design case. The results show that the counter-rotating vortex systems in tip cavity, as well as the interactions between leakage vortex and passage vortex, are significantly affected by the degree of wear damage. The starting location of wear and wear depth have pronounced influences on heat transfer and aerodynamic performance in squealer tip gap. After wear, the stage efficiency is decreased by about 0.3–1%, as the wear depth is equal to clearance gap size. In the serious worn case, thermal load on tip cavity floor is increased by about 60%, while the heat transfer on rims is reduced by about 20%. However, compared with the original design case, the area-averaged heat transfer coefficient on shroud is reduced by 5% at most.