The invention relates to an online analysis and decision method and system for extra-high voltage direct-current complex faults. The method comprises the following steps that: obtaining each categoryof data which operates in a power grid, carrying out integration on the data, and carrying out load flow analysis on the integrated data to simulate data change during power grid operation; carrying out security control analysis on the data subjected to the load flow analysis to simulate the complex faults, the operation way, the load flow level and the control strategy of a power grid direct-current system; and independently carrying out transient stability calculation, static safety analysis calculation and multi-infeed direct current short circuit ratio calculation on the data subjected tothe security control analysis, summarizing calculation results, and outputting the summarized calculation results. By use of the method, the accuracy of a direct current anticipated fault simulation result is guaranteed.