In order to measure the concentrated solar flux distributions of high concentration ratio, an infrared reflection measurement (IRM) method that bases on thermal infrared imager and water-cooled Lambert target is proposed, and the corresponding experimental apparatus are manufactured. Through the experiment at a sixteen-dish concentrator (SDC), clear infrared temperature images on the water-cooled Lambert surface are obtained. Then, the concentrated solar flux distributions of the SDC are simulated by the Monte Carlo ray-tracing method (MCRTM), taking into account the solar cone, the tracking error, and the slope error. By comparing with the experimental results and the simulation results, a slope error parameter of 2.5 mrad is detected. The conclusions can be used for measuring the concentrated solar flux distributions of high concentration ratio and finding out the slope error parameters of the solar concentrators.