To calculate the designed depth of large open harbor basin and channel waters，this paper takes a project in Ningbo as an example and analyzes the ridable tide duration acquired according to the influence of tidal level and current relationship and berthing and unberthing time.The paper calculates the designed depth using the specification formula and applies the safe surplus water depth stipulated by navigation practice to supplement the analysis rationality.The paper suggests that the ridable tide duration of large open harbor basin and channel waters can be determined according to the tidal level and current relationship，berthing and unberthing time，navigation requirements，and navigation organization scheme of the local marine area.The research method can provide ideas for similar port designs.