According to the working characteristics of over-voltage protective device for series compensation, the principle and structure of spark switch were introduced in this paper briefly. Special test loop and HV fast switch-off device were designed to evaluate the performance of spark swich in trials. Test data and mathematic statistic results revealed that the self-breakdown probability of spark switch obeyed the normal distribution and the cumulative distribution of spark switch self-breakdown had been less affected by different enviroment temperature. The number of trials was restricted to the characteristics of insulated gas and the electrode surface state in the practical application. Reasonable choice of test number can keep estimation error at a relatively low level, particularly for parameter estimation by using small sample of self-breakdown voltage. The conclusion is instructive to the development and engineering application of spark switch for over-voltage protective equipment of series compensation.