Based on the dynamic analysis of free-hanging risers, an improved Ant Colony Optimization (ACO) was proposed for optimizations of accelerations of equallength paths. Via the improved ACO, an optimized motion scheme for the upper end was obtained. Under the optimized motion scheme, the distance from the lower end to wellhead was effectively decreased and the re-entry took less time, which made the re-entry operation more convenient. In this paper, the different dynamic response and riser shape changes under optimized motion scheme and ordinary motion scheme were compared. The results demonstrate that the motion optimizations of the improved ACO is effective, and the optimized motion scheme obtained via improved ACO could make the re-entry more convenient.