In order to accurately analyze the left-turn delay of the borrowed left-turn design at intersections, a suitable delay optimization model is established to address the inadequacies of the existing delay models and to improve the scientificity of their settings. Firstly, the length of the reverse lane is optimized. Then, by analyzing the cumulative arrival/departure curves of the left-turning vehicles, a new optimization model considering the pre-signal delay is established with the objective of minimizing the total left-turning delay. Finally, a validation is carried out at an intersection on Furong Road in Changsha City, which analyzes the effect of the left-turn arrival rate on the delay of the borrowed-left-turn design as well as the effect of the length of the reverse lane on the delay of the left-turning vehicles. The results of the study show that: the larger the left-turn flow rate is, the more the advantage of the borrowed left-turn design can be fully utilized, and the left-turn delay can be reduced up to 59.21%; when the left-turn flow rate is low, the delay of the pre-signal position accounts for a larger proportion, and with the increase of the left-turn flow rate, the delays of the traditional and reverse lanes account for a gradual increase in the delay proportion. This study can accurately evaluate the practicality and advantages of the borrowed left-turn design, which is of great significance for the further promotion and application of the borrowed left-turn design.