Abstract: The primary factors affecting water environmental capacity for tidal river network,such as tide,storage capacity,water conservancy project,pollutants decay coefficients and boundary water quality of drawing water,were studied in this paper.Based on the water quality model for tidal river network,the computational method of aquatic environmental capacity was improved.The time-space changing law of the influence of above factors on aquatic environmental capacity was studied by the example of the river network of Punandong Area in Shanghai,and then the permitted pollutants discha rgeamount was put forward.