The invention relates to an automatic speed-changing bicycle. The automatic speed-changing bicycle comprises a bicycle body, wherein the bicycle body comprises an automatic diameter-changing chain wheel mechanism; the automatic diameter-changing chain wheel mechanism comprises a chain wheel sheet mounting plate and a wheel plate; sliding rods are uniformly distributed on the chain wheel sheet mounting plate in the radial direction; a spring and a sliding block chain wheel sheet component are sleeved on each sliding rod; the sliding block chain wheel sheet component consists of a sliding block, a chain wheel sheet, a bearing and a threaded pin; the sliding block is sleeved on the sliding rod; the chain wheel sheet is matched with a bicycle chain; the bearing is used for bearing the action force of the wheel plate; the threaded pin is used for connecting the sliding block, the chain wheel sheet and the bearing; guide chutes are distributed on the wheel plate uniformly; the bearing is placed in each guide chute; when the wheel plate rotates, the sliding block chain wheel sheet component is driven by the bearing to rotate; when an applying force during riding is changed, the bearing slides along the guide chute, and the diameter of a chain wheel consisting of the sliding block chain wheel sheet components is changed, so that automatic speed changing is realized. The automatic speed-changing bicycle adopts a pure mechanical mechanism, and has a simple structure, is low in manufacturing cost and has a good using effect.