There are stability risks after PV plants drop into island operation,especially in casa of unintentional islanding caused by grid faults and the islanding-state detection device fails.In order to estimate those risks,firstly,the paper theoretically analyzes the influence of three factors (power difference,PV permeability and motor inertia) on the stability characteristics of the island system in a short time after PV unintentionally dropped into island operation.Then the electromagnetic transient model of the PV station is established.According to that,the modified Dali Grid integrated with a PV station is modeled in PSCAD/EMTDC,verifying the theoretical analysis conclusions by simulation.Finally,quantitative stability risk index and method about PV station dropping into islanding are proposed according to the simulation results,which provide the theoretical basis for the stability risk estimation.