A control mechanism of the adjustable seat post for a bicycle is fixed to the bottom end of the lower outer tube of the seat post. The control mechanism is managed by a controlling wire to drive a pin within the seat post to move axially to adjust the position of the seat post supporting the seat, so that the controlling wire will not move up and down with the seat post for seat height adjustments. Thus, the control mechanism will not generate a residual wire occurring in a prior art seat height adjusting device. It is because the controlling wire of the prior art device is disposed to the top end of the seat post. Therefore, the drawback of a prior art device can be ameliorated by arranging the control mechanism to the bottom end of the lower outer tube of the seat post.