High precision angle measuring unit, which is the key to ensure accuracy and performance of rotary equipments, is extensively applied in measuring and tracking apparatuses. In regard to large-scale coordinate measuring instruments, angle measurement is the bottleneck to enhance instruments′ coordinate measurement accuracy by comparing with distance measurement. High accuracy cylindrical grating and four reading heads were adopted to build angle measuring unit for precise one-dimension turntable platform, then the influence on angle measuring accuracy by sensor itself, installation and axis shafting were analyzed in detail. Angle measuring error was calibrated by standard device and analyzed with harmonic method. A parameter optimization method based on genetic algorithm was proposed to develop error compensating model, by which angle measurement error was compensated. The experimental results show that cylindrical grating angle measuring error is reduced to ±0.7″ after compensation, which demonstrates that the compensation method is effective to improve accuracy obviously.