The invention discloses an adjusting mechanism for adjusting the handle bar position of a two wheeler or a three wheeler, comprising a handle bar part, an upper cover, a lower cover and a handle bar vertical pipe, wherein, the handle bar part is clamped, approximately horizontal and provided with approximately circular or polygonal sectional plane; the upper cover and the lower cover clamp the handle bar; and the top of the handle bar vertical pipe is provided with steps. One side of the upper cover and one side of the lower cover for clamping the handle bar are connected with each other, andthe other sides can be interlocked by an independent locking mechanism. As the lower cover is sheathed outside the handle bar vertical pipe and arranged below the steps at the top of the handle bar vertical pipe, the handle bar vertical pipe can be tightly fixed above a front fork by the locking mechanism, furthermore, the handle bar position can be repeatedly adjusted without influencing rigiditybetween the front fork and the handle bar, thus the two wheeler or the three wheeler has better comfort level and safety performance. Meanwhile, by loosening the locking mechanism, the whole handle bar system can rotate relative to the vertical pipe. When the mechanism is out of service, the handle bar can be rotated downward to the direction parallel to bodywork, thereby reducing space occupiedby bicycle.