To obtain higher thermal efficiency, advanced gas turbines operate at a turbine inlet temperature (TIT) up to 2000K, which causes an extremely high thermal load on the first stage vane endwall. Therefore, it is critical to understand the complex thermal environment near the endwall and develop efficient cooling schemes. In this study, adiabatic cooling effectiveness measurements on an annular cascade endwall with purge flow were conducted using a high-resolution infrared thermography technique. Effects of profiles and locations of an upstream slot on endwall film cooling behaviors were investigated at three coolant-to-mainstream mass flow ratios and an engine-like density ratio. Using numerical simulation, the flow details were illustrated to support the measured cooling effectiveness. Results indicate that moving the slot to a further upstream location will reduce film cooling effectiveness on the endwall. However, a further upstream slot can improve the uniformity of the film cooling effectiveness distribution. Compared with the normal slot, the convergent slot accelerates the coolant ejection to a higher momentum and thus inhibits the influence of secondary flows on endwall film cooling. Therefore, the convergent slot significantly improves both the coolant coverage and local film cooling effectiveness for all mass flow ratios and slot locations investigated. Quantitatively, an enhancement of approximately 25% in laterally averaged cooling effectiveness can be achieved by the convergent slot. •Measurements of adiabatic cooling effectiveness on an annular cascade endwall were conducted.•Effects of profiles and locations of the slot on endwall film cooling effectiveness were investigated.•The relationship between endwall secondary flows and film cooling behaviors was discussed.•The numerical results were used to reveal the flow details and film cooling mechanism.