Abstract: A 3D surface roughness measuring instrument is studied in this paper. A single bridge differential inductive sensor is used to detect the surface information in this instrument. A inductance signal processing circuit based on AD598 chip is designed, which have has good adaptability and performance to different inductive sensor signal processing The three-dimensional surface roughness evaluation software is developed in the C++ Builder development environment. The two-dimensional and three-dimensional roughness of the standard block is measured by this instrument.The measurement results are accurate and reliable. The instrument can evaluate the three-dimensional roughness of workpiece and has a good application prospect.