The inner refractive index distribution and absolute thickness of sample can be accurately measured by transmitted laser heterodyne interferometer. The combined optical scanning configuration composed of micro-electro-mechanical system (MEMS) mirror and f-θ lens is proposed, which is applied for the transmitted laser heterodyne interferometer system. The f-θ lens set is composed of three lenses with different materials, and has an approximately 40% transmission coefficient and a scanning area of 300 mm2. The f-θ lens model is optimized from the aspects of diffraction spot, distortion coefficient and beam divergence angle. The optimization results show that the series beam out of f-θ lens is perpendicular to the sample surface, and the system has a focal length of 434 mm and a distortion coefficient of 0.5%. The configuration for f-θ lens is presented. The result of optical scanning on the first surface of f-θ lens is introduced by He-Ne laser, which meets the requrements of the design target.