Understanding the seasonal variation and the mechanism in marine bay is helpful to decision-making of pollution control practice. This paper analyzed the seasonal variations of Cu in Jiaozhou Bay during 1982-1986. Furthermore, the mechanisms of the seasonal variations were analyzed. Results showed that the variations of Cu contents in spring, summer and autumn were relying on the source inputs of Cu. As a whole, the change process of Cu contents in Jiaozhou Bay were determined by the terrestrial transport process and oceanic transport process jointly.