This study focuses on the muddy-coastal headland bay beaches in Zhejiang Province, identifies the primary factors influencing the position of the sand-mud boundary, and analyses the transition mechanism. A depth formula of the sand-mud boundary point in headland bay beaches is proposed. The results show that: the tide level, wave height, beach slope and spring mean tide range in front of the beach are the main factors affecting the location of sand-mud boundary; the depth of sand-mud boundary is positively correlated with the 1/10 wave height in front of the beach and the spring mean tide range, and negatively correlated with the slope of the beach; the position of the sand-mud boundary exhibits seasonal and inter-annual variations. During summer, the sand-mud boundary tends to shift seaward and decrease in elevation, while in winter, it moves landward. Notably, in 2020, compared to 2019, the sand-mud boundary showed a clear trend of seaward migration and a decrease in elevation.