Abstract: As to tidal estuary hydrology and water quality elements have period time-varying characteristics in a day and a month, the computing method and evaluating standard for steady flow is not suitable for tidal estuary to control the water quality up to standards, so it cannot scientific verify the water environmental capacity for water pollution prevention and management. This paper aimed at tidal estuary unsteady reciprocating flow and time-varying water quality characteristics, proposed the timing and spacing double water quality control criterion, and then verified the environmental capacity of each functional area based on Qiantang estuary tidal water quality mathematical model. Time control criterion used two indexes to control which are the number of hours in a day and the number of days in a month of which water quality exceeds standard; space control criterion adopted four indexes to control which are average water quality of functional areas, water quality of cross sections, area of mixing zones and water quality of sensitive points. The water quality timing and spacing double control criterion can more comprehensively and truly reflect the status of water quality reach the standard of tidal estuary, so can effectively protect the tidal estuary water resources and water environment quality. The computation thoughts and methods can be referenced by environmental researchers and managers.