The invention discloses a preheating device and a preheating method of a printed circuit board. The preheating device comprises an outer casing, a heating assembly, a volute, a motor, an air wheel and a spray nozzle plate, wherein the outer casing is provided with an inner cavity, the heating assembly is arranged in the outer casing and is used for heating air in the outer casing, the volute is fixedly arranged in the outer casing and is provided with an air inlet and an air outlet, the motor is fixedly arranged on the outer casing and is provided with an output shaft which extends into the volute, the air wheel is arranged on the output shaft and is rotated with the output shaft to enable the hot air to be delivered into the volute through the air inlet and be sprayed out through the air outlet, and the spray nozzle plate is detachably arranged on the outer casing and is used for spraying the hot air sprayed from the air outlet to the printed circuit board. As the preheating device is provided with the detachable spray nozzle plate, and the size and the shape of a spray opening of the spray nozzle plate can be customized according to the conditions of the printed circuit board, the preheating device is suitable for the preheating treatment of various types of printed circuit boards, the application capability is high, component devices on the printed circuit board are furthest protected, and the safety and the reliability are realized.