Abstract: Cuijialu Bay is a unique circular bay in Qingdao, China. Beach profiles, historical maps, and ²¹⁰Pb/¹³⁷Cs dating of sediment cores from the beaches and muddy tidal flats located along the western coast of the bay were used to study the changes that have occurred to the bay and the relationship of those changes to the construction of a harbor. The landward beach is composed of a narrow swath of coarse sand with a steep gradient. The muddy tidal flats are relatively wide with a gentle gradient and consist of muddy silts. These muddy tidal flats occur only in the middle of the western coast and are characterized by fine sediments with a depth of as much as one meter. ²¹⁰Pb/¹³⁷Cs data show that these muddy sediments have been present since the 1950s and have accumulated at an average rate of 15 mm/a. In a core of 1.0 meters, the upper 0.7 m of muddy sediments is deposited under a sedimentation rate greater than that of the lower 0.2 m. It suggests that muddy sediments have accumulated more rapidly during the last 35 years since 1975. That date corresponds to the construction of a dyke, which reduced the width of the bay′s mouth by 30% and has very likely increased the siltation rate in the bay. OSL ages and ¹⁴C ages show that the river mouth bar was at least formed since 6.1 ka.