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Quasi-labyrinth seal technique was innovatively adopted to control the slashface flow, and the effect of slashface leakage on endwall aerothermal performance and blade suction side surface phantom cooling was numerically investigated. Simulations of five different labyrinth seal lengths ( Ls) and three coolant momentum flux ratios ( I) were conducted by computationally solving the Reynolds-averaged Navier–Stokes equations coupled with the shear stress transport k-ω turbulence model. The results show that the slashface coolant can be accelerated and conveyed downstream inside the slashface carried by the ingested mainstream and passage vortex. By setting up the quasi-labyrinth seal composed of a series of aperture cavities along the slashface, the coolant downstream transportation can be largely weakened, and the labyrinth seal with various lengths all have the potential to move the endwall coolant coverage toward upstream. When I = 0.48, the coolant downstream migration is severest and it has the best coverage on the endwall with seal. The laterally averaged cooling effectiveness is elevated for the fore part endwall but decreased for the back part endwall after sealing, and the highest cooling effectiveness is increased by 5% for 0.6 Ls seal. The 0.6–1.0 Ls seal has the approximate effect and is better than 0.4 Ls seal, and 0.6 Ls seal has the best outcome considering the countering effects of structural strength and endwall cooling performance. For 0.6 Ls sealed case, the high heat transfer level caused by coolant attachment is enhanced by 3.5% when I = 0.48 but decreased by 3.3% and 4.4% when I = 0.96 and 1.42 compared with baseline case. The low heat transfer caused by horseshoe vortex separation is enhanced when I = 1.42 for the sealed case. The quasi-labyrinth seal mainly weakens the back part suction side phantom cooling performance, and the well phantom cooling region moves upstream with the increase of I. The averaged phantom cooling effectiveness is decreased by 0.56%, 0.3%, and 1.44% when I = 0.48, 0.96, and 1.42 for the sealed cases. The results provide the gas turbine designers a better insight into improved slashface leakage control as well as its detailed surface cooling effects.