Zn is an essential substance for the metabolism of marine algae, and its biogeochemical cycle in coastal waters needs to be clarified by an appropriate biological indicator. In this study, the distribution characteristics of Zn²⁺ on the surface of alga Ulva lactuca L. (externalization) and alga utilization (internalization) were analyzed through simulating. The results showed that during the initial cultivation period (0 h–72 h), the Zn²⁺ enrichment of Ulva lactuca L. was mainly externalization. As the culture time was prolonged, this behavior gradually turned into internalization. The final internalization amount of Zn²⁺ was 10.35 μg·g⁻¹, and the externalization amount was 9.14 μg·g⁻¹. Ulva lactuca L. exhibited apparent Zn enrichment behavior. The adsorption capacity in the system had a linear relationship with the concentration of Zn and total organic carbon (TOC) in the solution. The linear relationship became gradually stronger with cultivation time. In summary, Ulva lactuca L. can become a crucial biological indicator for monitoring coastal Zn under natural conditions.