Given that the conventional five-arc trajectory design of the double-step horizontal wellbore may have no feasible solution in the case of limited horizontal section spacing，this study proposed a wellbore trajectory design method based on constrained optimization. By establishing a constrained optimization model to minimize the horizontal section length，an efficient solving algorithm was constructed，which combines the quasi-analytic solution and the bisection method. Given the failure problem that the singularity polynomial of the proposed quasi-analytic solution is always zero when the inclination and azimuth angles of the two horizontal sections are the same，a modified analytical calculation formula was proposed to deal with this theoretical defect of the traditional method. Compared with the traditional iterative method，the proposed method reduces the computational complexity to the order of solving quadratic equations via the characteristic polynomial dimension reduction strategy. Examples showed that this method can converge rapidly in cases of three-dimensional stepped wells，two-dimensional wells and simplified cases. By optimizing the adjustment of the horizontal section through the bisection method，the control of the minimum trajectory length under the constraint of wellbore curvature was achieved. This method can effectively deal with the trajectory design in cases of narrow spacing，like fault-separated reservoirs，and provide theoretical support for the development of drilling software. The research findings significantly improve the reliability and engineering applicability of the trajectory design for stepped horizontal wells.