The invention discloses a single-rack transmission bicycle capable of performing reciprocating variable circular motion. A transmission mechanism of the single-rack transmission bicycle can be used for bicycles, tricycles and other machines capable of achieving transmission through racks, and the single-rack transmission bicycle comprises a left pedal, a right pedal, a left pedal crank, a right pedal crank, a left pedal and right pedal alternating device, a single rack, a reciprocating variable circular device, a speed changing device and a reversing device. The reciprocating variable circular device converts reciprocating motion of the rack into circular motion, the speed change device can achieve high and low speed switching of the circular motion, and the reversing device can achieve reversing without matched actions such as lifting the rack. The bicycle is characterized in that up-and-down motion of the pedal is converted into reciprocating motion of the rack, and then the reciprocating motion is converted into circular motion efficiently with low noise through the reciprocating variable-circumference device; the gear and the rack can be sealed, so that oil stains are reduced, and sufficient lubrication can be realized; the problem of chain falling is avoided; a single rack is equivalent to a large fluted disc of a traditional bicycle, and the stepless speed change function can be achieved.