The present invention relates to a bicycle seat post structure, and more particularly, to a structure provided with a hydraulic actuator within which is configured with an upper oil compartment, an intermediate oil compartment, and a lower oil compartment; the three oil compartments intercommunicate. It is characterized in that the bicycle seat post structure provides an upper valve moving simultaneously with the pin and configured between the upper oil compartment and the intermediate oil compartment; the upper valve will be closed to disconnect the upper oil compartment from the intermediate oil compartment when the pin is in a state of locking. Thus, the valve will stop moving up because it is positioned above the upper and the intermediate oil compartments and consequently controlled by the oil within the compartments, so as to stop the seat sliding upward and further to be combined with the bicycle frame as a whole.