The invention provides an online detection method and system for a GIS operational condition. The method comprises the steps as follows: firstly, a relative deviation value between a characteristic value of a GIS and a standard characteristic value is computed; then a space weight coefficient array is defined according to the characteristic value; a corresponding weight coefficient is selected from the space weight coefficient array according to the relative deviation value; the relative deviation value is multiplied by the weight coefficient corresponding to the relative deviation value to obtain an individual deviation value; the computing process is repeated by selecting different detection moments so as to obtain a plurality of individual deviation values; and finally, the individual deviation values are accumulated to obtain a space distribution statistical value, and the GIS operational condition is confirmed according to the space distribution statistical value. The weight-containing detection analysis is adopted, and the condition that potential faults of the GIS can be effectively revealed by historical data with severe deviation degree is considered, so that the current GIS operational condition can be accurately reflected.