In allusion to the grid structure and operation characteristics of the microgrid, in this paper a static voltage stability index for microgrid was deduced based on the existence of the power flow. The index has taken the node voltage into account and is free from the influence of the grids R/X; besides, it has a smaller error for it has no network loss equivalent during its deduction. Then one 13-node microgrid system was established to validate the deduced index, finally the index was used to analyze the influence of distributed sources including the PV, PI, P-Q(V) node types as well as heir locations and capacities on the static voltage stability of microgrid.