Abstract: The optimization of trajectory for stepped horizontal wells takes well design depth or friction as the target function,which is only based on techniques.It hardly takes economy and the safety of drilling into account.An optimal method of designing well trajectory for stepped horizontal wells is introduced in this paper.The principle of designing well trajectory features in safety,utility,and the lowest cost.Based on the design principle of well trajectory for stepped horizontal wells and the application of the mathematic optimization theory,a target function of optimal well track is established to make the cost the least.This is a nonlinear target function with constraint,which can be solved by using the exterior penalty function method.Then the optimal well track can be obtained.Field experiment shows that the method presented in this paper is correct and reliable,which can fulfill the field requirement.