The utility model relates to a portable bicycle, whose rear fork is hinged with the rear end of a sleeve tube of the crossbeam; the rear fork can rotate retractably by 110 DEG around the hinging point, and the sleeve tube can expand and contract freely in the rectangular section tube of the crossbeam. The handle bar adopts a movable adjustable structure whose opening degree and shape can be adjusted arbitrarily within a certain range, and the complete bicycle envelope size of the portable bicycle can be shrunk to 690 (length) *690 (height) *220 (width) millimeters to the minimum. The transmission mechanism of the portable bicycle is composed of an increasing speed changer, a nylon belt and a pair of flywheels, which can effectively convert man power into the motive power of the portable bicycle, with no need of oil's being added for lubrication. The buffer and the damper which are installed on the rear fork and the hinging site of the bicycle frame can lower the vibration and bump of the moving portable bicycle, which makes people feel brisk and comfortable.