Abstract: The mechanical shim is an advanced technology for reactor power control and axial offset control upon the regulation of control rod assemblies. The variable positions of control rod assembly adversely impact the accuracy of the traditional axial offset control based on the calibration with ex-core power range neutron fluence rate measurements. The signals from in-core self-powered vanadium detectors which provide very precise measurement of neutron fluence rate distribution normally can be utilized for the tuning. To compensate the unacceptable long response time delays, a lead-lag controller was designed and the method of the axial offset control upon the compensated signals was proposed. The simulation results indicate that the method can improve the accuracy of the axial offset control by overcoming the negative impacts resulting from the movement of the control rod assembly. The further analysis, in terms of safety, applicability and performance issues, indicates the design is of great value.