The invention provides a bicycle gear transmission mechanism. The bicycle gear transmission mechanism comprises a double-ratchet rear wheel, pinions, fan-shaped inner gears, a rack, a fixed pulley, pedals and a rope. The two pinions are arranged on the double-ratchet rear wheel and then are respectively meshed with the fan-shaped inner gears. The fan-shaped inner gears are arranged on mandrels on two sides of the lower part of a frame. The pedals are installed on lever arms at the front ends of the fan-shaped inner gears, the length of a force arm can be changed by stepping on different positions of the pedals, and speed changing is achieved. The lever arms at the front ends of the two fan-shaped inner gears are wound around the fixed pulley installed on the rack through the rope to be connected together, and pedal linkage is guaranteed. Circumferential motion is replaced by vertical reciprocating motion during riding, work can be done by means of the gravity of a human body, the force exerting function of the human body structure is exerted to the maximum extent, unnecessary force exerting of the human body is reduced, the comfort of a rider is improved, the number of parts is small, design is reasonable, machining and maintenance are easy, transmission protection and sealing lubrication are easy, and large-scale popularization is easy.