Spot distortion caused by astigmatism effect in orthogonal cylindrical lenses is utilized to measure axial displacement with 30 nm resolution, strong noise immunity, and compact experiment setup. Axial displacement of the sample surface is determined by four-quadrant difference processing of distorted laser spots' energy distribution images received by CCD. Four-quadrant difference processing results indicate an applicable measuring range of 5.6 μm (cubic fitting r=0.9988) with a highly linear range of 1.2 μm (linear fitting r=0.9996). Factors affecting measuring range and sensitivity are analyzed by theoretical deduction and numerical simulation. This technique has potential applications in drifting sample tracking and measurement in advanced microscopy.