Laser-projection displacement meters (LDM) are widely used for bridge deflection monitoring. A comparative calibration method is proposed to fulfill the LDMs’ on-line and in-situ calibration requirement. Taking a laser displacement meter with traceable calibration as a reference sensor (RS), the in-situ measured value series by the LDM under calibration (SUC) and the RS are firstly acquired simultaneously under the circumstances of the same time-varying excitation source. Then, by matching the two series based on feature points sectioning method, the phase differences caused by the variation of excitation source and diverse response characteristics of sensors are solved, and sequentially comparative measured values with respect to the certain inputs are obtained. Finally, performing the statistical analysis to the matched data series produces the calibration results of the LDM under calibration. An on-line calibration simulation system is designed and set up according to the real in-situ conditions, accompanying with a verification subsystem using laser interferometer as a reference to verify the validity of the proposed method. The experimental results show that the proposed method has a high consistency with the method of simultaneously comparative measurement by using laser interferometer as a reference. Especially, the proposed method can be used under in-situ conditions of bridge deflection monitoring application.