Focus detection method is one of non-contact profile measurement methods. However, the measurement accuracy of current focus detection method is limited by voice coil motor adopted by it. In this paper, based on an improved Foucault focus detection method, a new non-contact displacement sensor with diffraction grating metrology system is presented. Driven by a piezoelectric actuator instead of a voice coil motor, and a diffraction grating metrology system being with it, the sensor has high measurement accuracy. During surface profile sampling, according to focusing deviation signal, the focusing lens was driven to move vertically by the piezoelectric actuator so that its focus was always located on the workpiece surface, synchronously the vertical displacement of the focusing lens was obtained by the diffraction grating metrology system as the profile height of sampling points. The displacements of all sampling points gave the whole profile of the measured surface, which can be processed by a characterization software to obtain the measurement result. The resolution of the non-contact displacement sensor was 10 nm.