Wind-thermal bundled power transmission system with series compensation is considered as a feasible way for solving consumption problem of large-scale renewable energy resources. But interaction between wind turbine, thermal power unit and power grid may lead to sub-synchronous resonance, which will cause irreversible damage to thermal power unit. This paper discusses two feasible analysis methods for sub-synchronous stability and damping characteristics of thermal power unit in wind power bundled system with series compensation. Principles, advantages and disadvantages are theoretically analyzed, and analysis results of these two methods are compared based on Shangdu wind-thermal power transmission system.