In order to assess the voltage stability as wind power is integrated into the power system, a method is proposed to analyze probability characteristics of voltage stability based on probabilistic loading margin index. The deviations of these random parameters are small such that linear approximation of loading margin with respect to these parameters is valid. After calculating the sensitivity matrix of loading margin with respect to random parameters, superposition principle is applied to separate the loading margin deviation which varies around the expectation into two parts: load random parameters and wind random parameters respectively. The simulation results of the proposed method compared with those obtained from Monte Carlo method and moment and cumulate method for IEEE-30 test system show that the proposed method is not only precise but also simple and fast.