The main canal of the Middle Route South-to-North Water Transfer Project is so long as 1241.2km,and has hundreds of cross-structures such as aqueducts.If any one fails somewhere on the canal due to ab-normal floods, water supply will be discontinued.Therefore,the hydrological risk is one of the key points in the re-liability and risk assessment for the project.The risk computation for the complex serial system is effectively sim-plified by two-dimension joint probability distribution and the Minimum Frequency Series.A general risk asses-ment model has been presented and calibrated comprehensively with the data of a segment of the canal.It turnsout that the results are reasonable and the model is applicable for the other water supply systems.