This study investigated the transient natural convection phenomenon in an underground water pit thermal storage with heat losses through the surrounding walls. An experimental test rig was built up, and a numerical model was developed to obtain the characteristics of the thermal stratification in the water pit thermal storage. Fluid properties are assumed to be constant, except for the density changes with temperature which is treated using the Boussinesq approximation. The simulations of temperature profiles are reasonably proved by experiments with the maximum relative errors of +/- 9.77%. The results show that water temperature decreases close to the walls due to the heat losses, which leads to a downward flow along the inclined sidewalls. A slight upward flow occurs at the center of the water pit thermal storage, which lifts the warmer water to a higher level. As a result, the buoyancy-driven flow gradually builds up the thermal stratification in the water pit thermal storage. The modelling results also show that the values of the average Nusselt numbers on the inner surface of the inclined sidewalls and the bottom of the tank are much higher than that of the top thermal insulation layer. The thermal energy storage efficiency decreases rapidly in the first five minutes from 100% to 83.19% due to the intense heat transfer, and then tends to level off at the end of 40 min. The maximum velocity of the natural convection appears close to the upper part of the inclined sidewalls, and it decreases with the cooling process evolved. The present work has a valuable attempt to fill the gap in the existing studies and is useful for guiding the water pit thermal storage design.