In the measurement field of micro/nano devices, many instruments have a contradiction between high resolution and large measurement range. This paper developed a non-contact optical nanometer probe based on the astigmatism method. This probe employs a high numerical aperture (NA) focusing lens to achieve nanometer resolution of the longitudinal measurement. Meanwhile, a longitudinal scanning measurement method based on two focus error signal (FES) curves is proposed to break the limit of low measurement range caused by high NA value. Therefore, it enables the probe to achieve the nanometer resolution of longitudinal measurement and large measurement ranges. The experiment results show that the longitudinal measurement resolution of the proposed probe is better than 1 nm, and the repeatability error is 17 nm. Combined with the high-precision microstage, the longitudinal range is greater than 15 μm. The proposed probe can be used for large-range longitudinal non-contact measurements with nano-scale measurement requirements.