In this article, one new non-contact displacement sensor and its principle has been introduced; it is based upon improved Foucault focus detection and equipped with diffraction grating measuring system. Driven by piezoelectric actuator instead of voice coil motor and diffraction grating metrology system being adopted, the non-contact displacement sensor avoids non-linear error and other measurement error caused by the movement of the voice coil motor and therefore has higher measurement accuracy, which has be proved by series of experiments. During measuring the workpiece surface contour profiles, piezoelectric actuator drives focus object lens to make vertical displacement, and ensures focus in every sampling interval to go to the workpiece surface. In the mean time the focus error signal can be set to zero. The sensor has large range and high resolution, which can be applied in profile measurement. This non-contact displacement sensor resolution can reach 10nm.