Abstract: The ecological status of rivers is determined mainly by pollution levels, which are increasing under conditions of enhanced low flow. It is therefore very important to determine those volumes of low flow that will maintain good water quality, such flows can be termed water requirement for the stream's self purification. Based on the water environmental function division of the river, the water quality should meet the stream's environmental quality standard of the surface water so as to keep in good utilization. If only considering water pollutants from point source, the river can be divided into several reaches marked by each sewage discharge port. Taking into account of the degradation of pollutant substance along the stream, it proposes that minimum stream flow requirement of each should be ascertained according to the environmental quality standard of the surface water, the instant it is mixed with the pollutants at the discharge port. Then, one of the calculation methods of water requirement for the stream's self purification is set up. It further considers that water requirement for the stream's self purification of the study zone should choose the maximum among the reach's self-purification water requirements so as to meet water quality standard of each reach. Using the method, the water requirement for the stream's self purification of the Weihe River is calculated on the basis of BOD₅ influx along 85 point source discharge ports. The investigative spatial scale is instream inside; the temporal scale is multi-even year (from year 1995 to 2000) and representative years which include P=25%( year 1963), P=50%( year 1990), P=75%( year 1982), P=90%( year 1979) and even every month of each year. The result shows that the water requirement for the stream's self purification varies little with annual stream flow, the degree of water requirement shortage for stream's self purification is severe, most of which appears in non-flood season. Estimated by even flow year (P=50%), the water requirements for the stream's self purification of Linjiacun, Weijiabao, Xianyang, Lintong and Huaxian sections are 29.4?10⁸ m³/a,34.93?10⁸ m³/a,54.80?10⁸ m³/a,45.00?10⁸ m³/a,42.09?10⁸ m³/a respectively as sewage discharge at present, while that are 10.79?10⁸ m³/a,13.19?10⁸ m³/a,31.24?10⁸ m³/a,26.57?10⁸ m³/a,31.04?10⁸ m³/a as sewage inpour at discharge standard. Compare sewage inpour at discharge standard with that at present, on the one hand, the water requirement for the stream's self purification declines in the gross according to the pollutant discharge concentration decrease as sewage inpour at discharge standard. On the other hand, the degree of water requirement shortage for the stream's self purification is still severe as sewage inpour at discharge standard, which appear in January, February and December.