A v-groove structure is extensively used for planar optical-fiber alignment because of its unique features. To obtain low connection loss, the core pitch tolerance of a v-groove structure should be within submicrons. The squareness errors have great influences on the machining precision of micro v-grooves. Therefore, the actuarial calculation of squareness errors is of great significance for ultra-precision micro v-groove machine tools. In view of the disadvantages of the squareness error analysis method basing on small angle error assumption, the actuarial calculation of squareness error is implemented. A more precise geometric error model basing on the actuarial calculation of squareness error is established. The transformation between actuarial calculation of squareness error and improvement of machining precision is achieved through the geometric error modeling. The results show that the actuarial calculation of squareness error is effective and practical.