A bicycle seat air pressure rise and fall adjusting device comprises a piston rod. A fixed device and an air pressure device are arranged in sequence at the lower part of the piston rod; the fixed device can be tightly clamped on the piston rod or separated from the piston rod; the air pressure device comprises an air pressure cylinder; a piston and a compression spring are arranged in sequence in the air pressure cylinder from the top to the bottom; the upper end of the compression spring is fixedly connected with the piston, the lower end of the compression spring is fixedly connected with the bottom of the air pressure cylinder; a switch rod is arranged on the side wall of the air pressure cylinder; the middle part of the switch rod is connected with the air pressure cylinder through a rotation shaft, the bottom of the switch rod is connected with the air pressure cylinder through the spring; the bottom end of the switch rod can be inserted into an air hole formed in the lower end of the air pressure cylinder; the switch rod is connected with the upper end of the piston in the air pressure cylinder. A circle of rubber layer is arranged across the bottom end of the switch rod, and thus the air hole can be sealed well in a manner that the bottom end of the switch rod is inserted into the air hole. By adopting the structure, the purpose of adjusting the height of a bicycle seat can be attained.