Abstract: Site-selection and designing of a tidal stream energy station usually consider not only tidal stream power density,but also water depth,current direction,and velocity profile. This paper investigated the relationship between surface velocity and vertical distribution of power density and introduced the Direction Stable Coefficient to describe the variability of surface flow direction. Tidal Current Energy Index,which is based on water depth,maximum speed during flood and ebb,and rated velocity of the tidal turbine,is proposed for characterizing the potential utilization value of the tidal energy in a site. This method is then applied to the Putuo islandHulu island channel of the Zhoushan islands. The index can provide a scientific reference for macro-siting and initial design of tidal stream plants.