A pressurized coolant reservoir tank assembly may include a tank having an injection port formed at an upper portion thereof, storing coolant for cooling an engine of a vehicle therein, and having a discharge hole formed to penetrate a side surface of the injection port such that steam or coolant is discharged to an outside through the discharge hole, a pressure cap fastened to the injection port such that the discharge hole is opened when the pressure in the tank reaches a predetermined pressure or the pressure cap begins to be rotated, and a burn prevention cover fitted to an outer surface of the tank with a gap between a bottom surface of the burn prevention cover and a surface of the tank such that the steam or coolant discharged through the discharge hole flows downward of the tank along the surface of the tank.