The design water depth is the primary factor in the site selection of catenary anchor leg mooring（CALM）. The current codes by the Classification Society provide a method for determining the design water depth with high requirements for basic data such as ship size and environmental parameters. Hardly are they applied directly in the early design stage of the project. This paper studies the motion response of mainstream moored ships in different environments by numerical analysis. It proposes the coefficient of design water depth based on the draught of moored ships which can directly calculate the minimum water depth. Furthermore，the calculation is compared with the test results of the physical model and existing engineering cases to show the reliability and applicability of the coefficient. The proposed coefficient and calculation method for the design water depth can provide a reference for the site selection of CALM in the early stage of the project.