Abstract: Aiming to investigate seasonal variations of sandy-muddy transitional beach geomorphology and the key controlling factors on Baicheng beach of Xiamen in 2022-2023, the responses of surface sediment grain-size to the beach morphological variations were studied. Results showed that Baicheng beach was strongly eroded windward and accreted in the other section from autumn to spring when it was mainly controlled by the clockwise longshore sediment transport surrounded Xiamen Island. In view of monsoon wind direction changes and the weakening of the waves, the longshore sediment transport could be reduced clearly in a state of weak beach erosion. From spring to summer in 2023, the longshore sediment transport weakened further, but all profiles of Baicheng beach showed different erosions with intensified sediment loss related to the seaward transport of sediments caused by the strong precipitation in summer. Meanwhile, tourist activities have also intensified the beach erosion. Then, these seasonal changes showing strong-weak-strong erosion intensity on Baicheng beach was related to the seasonal transfer of longshore and cross-shore sediment transport, and subsequently, the grain size of surface sediment presented some immediate changes. From autumn to the following summer, the profile slope and sediment grain-size of Baicheng beach decreased at first and then increased, i.e., the higher erosion intensity the steeper beach slope coupling with sediment grain-size increasing. It is mainly related to the seasonal transfer from longshore and cross-shore sediment transport and their interactions. The observations at Baicheng beach have provided some new findings for a better understanding of sandy-muddy transitional beach morphodynamics.