The invention discloses a grid observability analysis method for synchronous phasor measurement. The method includes the following steps that: step 1, the measurement islands of the initial synchronous phasor measurement of an entire power network are formed by means of PMU current phasor measurement; step 2, a PMU zero-injection measurement boundary node adjacency list is formed, the measurementislands are initially merged through the boundary node adjacency list according to synchronous phasor measurement topology merging rules; step 3, to-be-merged grids are formed, PMU measurement islandsassociated with qualified to-be-merged grids are merged according to to-be-merged grid merging rules, and observable PMU measurement islands are determined after the completion of the merging; step 4, as for to-be-merged grids which do not satisfy the to-be-merged grid merging rules, a numerical value method is adopted to search missed observable islands; and step 5, the observability of the network is judged, all observable islands constitute an observable area, if the area covers the entire network, the whole network is observable, otherwise, the whole network is unobservable, and the observable area is outputted.