The part’s surface quality of NC machining is influenced by the chord error greatly. The confined chord error algorithm for machining complex parametric surface is proposed for controlling the chord error. The arc length error is utilized to control the chord error of the interpolated point indirectly. The arc length error of interpolated point is computed by trapezia expressions, the coordinate and the first order derivative of interpolated point is computed by the interpolation algorithm. The computed error of confined chord error algorithm is discussed and the simulation indicate that the destined precision of the chord error can be satisfied by this algorithm.