The invention discloses a pollution source diffusion early-warning method based on a water quality on-line monitoring system. The method comprises the following steps of S01, monitoring water quality current situation of each monitoring station in an area in real time through the water quality on-line monitoring system; S02, confirming a pollution source being warned; S03, acquiring real-time monitoring data of excessive pollutants at the monitoring station where the pollution source is located; S04, selecting a river where the station is located to simulate a terminal; S05, inputting relevant parameters of a river section to be simulated; S06, calculating a one-dimensional steady-state water quality model; S07, rendering a pollutant concentration gradient for the simulated river section and carrying out early-warning information judgment. The one-dimensional steady-state model applied by the invention is independent of specific riverway geographic information and terrain parameters, only relates to hydrologic condition flux and flow velocity of riverways, can be automatically modeled according to GIS (Geographic Information System) information, and is strong in universality.