The invention provides an on-line environment monitoring system. The on-line environment monitoring system comprises a pollution source on-line monitoring subsystem, a main water area water quality automatic monitoring subsystem, an urban air quality monitoring subsystem, an urban noise monitoring subsystem and a monitoring center. The on-line environment monitoring system has the following advantages: the advanced development technology is selected and used, and the application system is simple and convenient to operate and is suitable for being operated and used by people of all levels; the advanced design concept is achieved, expansion can be conducted at any time according to development and demands of environmental management, and the on-line environment monitoring system is compatible with the present application system; the on-line environment monitoring system is favorably reflected in the aspects of network coverage, reliability, stability, real-time transmission speed, data transmission accuracy and safety, operation interface, reasonable construction investment and daily running costs and the like.