The square-shaped hot water storage tank is widely used in thermal energy storage systems due to its easy processing and installation. In this study, to address the degradation of water discharge efficiency caused by the mixing of cold and hot water in the hot water discharge process of square tanks, based on modelling experiments, particle image velocimetry and laser-induced fluorescence techniques were used to visualise and quantitatively analyse the thermal mixing phenomenon in a conventional square hot water storage tank with typical water discharge flow rate in engineering applications (940 ≤ Re ≤ 3290) and in the tanks with three different outlet positions. A detailed visualisation and quantitative analysis of the hot and cold water mixing phenomenon in the storage tank was carried out. The experimental results show that the maximum vortex height (hv) formed by the jet gradually increases with the increase of Re at the inlet, and the jet has stronger entrainment and mixing effects. Compared with the effect of Re, the jet-induced entrainment and mixing with the effect of different hot water outlet positions (h0/l) are relatively small. When the h0/l is 1/2, a noticeable temperature stratification occurs for 940 ≤ Re ≤ 1880,. For 2350 ≤ Re≤ 3290, the stratification weakens, and the mixing between the jet and the surrounding fluid becomes more intense, resulting in a broader distribution of the thermocline. The results of the study can provide a reference for optimising the energy-saving design of the square hot water storage tank.