To address the traffic congestion caused by left turn traffic flow at urban road intersections and to improve intersection capacity and safety, various design types of left-turn lanes were summarized. According to the current inadequacy in the design length of left-turn lanes at domestic intersections, typical intersections were selected to study the turning process of left-turn traffic during peak hours. By using kinematic equations and traffic flow fluctuation theory to estimate the maximum queue length extending upstream, a new algorithm for determining the length of left-turn lanes was proposed. This algorithm considers the transmission, delay, and continuity characteristics between vehicles. Using the design length calculated by the new algorithm can reduce parking delays caused by left-turning vehicles and enhance traffic efficiency at intersections.