To correct the measuring angle errors derived from the eccentricity of installing a circular grating in a manipulator joint test system, a model for compensation of grating measuring angle errors and the test method for installing eccentric errors are investigated. First, according to the relationship between angular error and eccentric parameters, the paper deduces the model of angular errors caused by install eccentricity. Then, it describes the method and principle to detect the grating eccentricity and eccentric direction by receiving phase errors of signals using two sensors in contrast. Based on the Lissajous plot generated by composite signals, the eccentric parameters of the grating in manipulator joint test system is measured. Finally, using the angle error formulas to compensate the measuring angle, the measuring system has been amended. The experiment shows that the amended angular accuracy has greatly improved, and the measurement accuracy is increased by nearly five times. It can satisfy the requirements of high accurate systems.