To realize a high-speed and a high-precision machining in the CNC system, an adaptive look-ahead control algorithm is proposed, which optimizes the transition velocity between trajectory segments. This algorithm adds the asymmetric S-curve acceleration and deceleration control as the path length constraint, which is based on the traditional velocity transition modes between trajectory segments. And it can adaptively make a plan for the optimal velocity between trajectory segments according to the changes of adjacent trajectory two segments length. An x-y CNC system rig of independent development is used to verify this algorithm. Experimental results show that the machining precision of the comprehensive processing can reach ±0.6 millimeter and its productivity is enhanced by 9.8%. It indicates that the proposed algorithm can avoid the problem of significant increase calculation when control system back to calculate the transition velocity. It also can reduce frequently start and stop of feed shaft during the process of a large number of tiny trajectory segments. Compared with the traditional look-ahead algorithm, the proposed algorithm can significantly improve the smoothness of transition velocity between the adjacent trajectory segments, the machining efficiency and precision.