The invention discloses an automatic calibration method of a water quality monitoring system with adoption of an ultraviolet absorption method. The automatic calibration method is characterized in that a standard solution colorimetric cuvette switching device is designed and installed on a measurement optical path so that multipoint calibration can be performed within the measuring range of the system, and corresponding algorithms are adopted in a processor to determine a coefficient of correction in order to realize automatic calibration. The method has the advantage of saving a large amountof manual operation, improving the accuracy and practicability of the system and providing technical support for long-term online monitoring of water pollution. In a standard solution comparison experiment, the relative standard deviation is not more than 1% and the maximum relative error is not more than 2%, thus relevant national standards are accorded with.