The mixing process in a 0.42-m diameter transparent stirred tank with improved double Intermig impellers was experimentally studied using the decolorization method with four different tracer injection points. The time dependence of the tracer concentration distribution was recorded using a high-speed camera. The turbulent flow and tracer dispersion were calculated by a large eddy simulation (LES) combined with the dynamic Smagorinsky-Lilly model. The tracer diffusion process, tracer concentration response, and mixing time in the tank stirred with the improved double Intermig impellers were investigated. The results showed that the mixing process predicted by LES was consistent with that by the decolorization method experiment. The predicted mixing time was 14% longer than the experimental time. The mixing time was the shortest when the injection point was in the middle of two impellers. The mixing time increased when the injection point moved to the top or bottom of the tank. The mixing times at the monitoring points near the injection point were shorter than those at distant points. The bottom of the tank with the improved double Intermig impellers was the crucial region that was difficult to mix. The mixing times at the monitoring points at the bottom were the longest for all the injection points. Increasing the mixing speed could promote the mixing process of the medium in the stirred tank and reduce the mixing time without changing the flow field structure. Therefore, the results could serve as a guide for the design and engineering application of such a stirred tank.