Based on the material balance theory,the quantitative relation has been established among substance endogenous load of nitrogen and phosphorus,their quantity of input and output of the reservoir,and the storage quantity in the water body.Take Dongzhang Reservoir of Fujian Province as an example,the quantitative analysis of the endogenous load of nitrogen and phosphorus in a reservoir was carried out,and its temporal distribution law was revealed.Furthermore,this paper expounds the method and procedure of how to reckon the endogenous load of nitrogen and phosphorus in the reservoir month by month according to the surveyed statistical data of water quality and hydrology.The research tells us that the distribution of endogenous load of nitrogen and phosphorus is subject to seasonal change and the level of endogenous load is somewhat concerned with dissolved oxygen in the water bottom.From January to April,it is oxygen-enriched in the water bottom,therefore,the input of endogenous of nitrogen outweighs the output and the load appears positive;in other months,it appears negative.In July and August,it is anaerobic in the water bottom,therefore,the input of endogenous phosphorus of outweighs the output and the load appears positive:in other months,it appears negative.This approach of analysis proves to be quite accurate with abundant and precise data on the input and output of pollutants,the inspection information of the accumulation of water pollutants and other concerned hydrologic information.