This paper proposes probabilistic Power System Health Index(PSHI). The paper describes several kinds of power system health indices based on two main categories, which are adequacy and security. In adequacy, four kinds of health indices of Frequency, Voltage(154kV, 345kV and 765kV), Reserve(Operating Reserve Power and Frequency Regulation Reserve Power) and Overload of lines and transformers are proposed. In security, four kinds of health indices of Voltage(154kV, 345kV and 765kV), Overload of lines and transformers, Power transfer limits among areas and Special Protection System(SPS) are proposed. All indices are mapped into three domains, which are Health, Margin and Risk, defined with expert interview. While definition of domain of health, margin and risk is similar with the conventional wellbeing analysis of power system, definition of the domain proposed in this paper comes from expert system based on expert interviews. The several kinds of health index functions, which are linear ratio, piecewise linear ratio and reverse ratio function etc, are developed in this paper.