a height adjusting device for a bicycle seat post provides an up - down adjust unit . all the oil - flow control elements of the height adjusting device are arranged inside the up - down adjust unit . the oil flows between the valve port and the oil - block plate are blocked by both the oil - block plate and the valve port touching tightly in an up - down engaging manner . by the structure , what repairing the oil - flow control elements needs is to replace the up - down adjust unit when the oil - flow control elements are out of work . therefore , it is convenient to repair and assemble . the valve port and the oil - block plate are arranged up and down so that they may touch each other closely in surfaces engaging . thus , it provides good oil - blocking effects . besides , the oil - block plate receives no shear stress occurring in the prior art generated from suddenly pressing down the seat . therefore , it has a long service life .