The invention provides a direct quenching device for a medium plate and a quenching process thereof. The device comprises a high-level water tank, a water quantity distributor, upper quenching coolers, upper accelerating coolers, upper fine-tuning coolers, lower quenching coolers, lower accelerating coolers, lower fine-tuning coolers, edge shielding devices, side spraying devices and a purging device. The quenching method comprises the following steps: carrying out controlled rolling, and controlling the finished rolling temperature at 780-850 DEG C; calculating the speed of a roller way, the group number of the coolers to be started, the water quantity of the coolers and the edge shielding quantity according to the plate thickness and the steel grade by utilizing a computer, and then transferring the plate to a controlled cooling area according to a predetermined speed; and starting the coolers and the corresponding side spraying devices according to the position sequence of the plate, wherein, the finished cooling temperature range is 150-450 DEG C. The direct quenching device has the characteristics of low fabrication cost, and fast and uniform cooling speed, and can stably produce the flat quenching plate with the quenching thickness of 9-60mm, and is simple and convenient in maintenance; and compared with the traditional out-line quenching method, the quenching process provided by the invention has the characteristics of short process flow and less energy consumption.