The present invention relates to a rudder resistance reducing method which utilizes a negative pressure to guide in gas for generating air bubbles, reducing the resistance of the rubber of a boat. The present invention uses an air bubble generator in front of the rudder for generating bubbles during rotation of the propeller of the boat to cause a high-speed flow of water without changing the original design of the boat, enabling the air bubbles to isolate contact between the high-speed flow of water and the rudder, thereby reducing rudder resistance and saving energy consumption.