The invention relates to an experimental device and a method for testing a discharged water temperature of a reservoir. The experimental device comprises a tail water mixing tank with a tail water tube outlet, wherein the tail water mixing tank is connected to a necking steady-flow tank which is connected to a water collecting tank; a mixing facility for enabling a water upper layer and a water lower layer to generate interflow is arranged at the downstream of the tail water tube outlet; a water temperature monitor is arranged in the water collecting tank; and the water temperature monitor is electrically connected with a water temperature data acquirer. According to the experimental device, the mixing facility is arranged in the tail water mixing tank, so that the discharged water flow can be mixed very well, and therefore, layered monitoring errors of the discharged water temperature are effectively solved. Besides, a multi-point type water temperature monitoring system is arranged in the water collecting tank, so that the water temperature condition of each layer in the water collecting tank is monitored in real time; and meanwhile, at a discharged water temperature testing area, discharged water temperature evaporative heat loss is reduced through a transparent heat insulating cover, so that the accuracy of acquired experimental data is guaranteed.