Based on the basic principle that back focal plane imaging can determine the transmission properties, a polymer planar waveguide optical parameter measuring instrument is designed combining with Fresnel reflection theory of planar waveguides. The real time measurement of the local refractive index and thickness of the sample with high accuracy and high spatial resolution is realized by fitting data with MATLAB. The measurement accuracy of planar waveguide thickness can reach nanometer scale, and the spatial resolution can reach 300 nm. The instrument is simple in structure and easy to operate. It has high application value in optical sensing and intracellular biological sensing fields.