Abstract: This paper mainly studied dynamic interaction between unified power flow controller（UPFC） and synchronous generator（SG） shaft caused by mode resonance. Drawing lessons from modeling theory of complex torque coefficient method, power system with a UPFC was divided into two independent and interconnected parts: SG shaft subsystem and main system with UPFC. Mathematical expression of UPFC control modes was derived. For the IEEE first benchmark system and a real 220 kV equivalent system with a UPFC, sub-synchronous control interaction between UPFC and generator shaft was analyzed from perspective of modal resonance. In order to quantitatively analyze effect of resonant phenomenon on system oscillation stability, a new method of modal resonance was proposed to approximate position of two closed-loop modes in complex plane when sub-synchronous modal resonance occurs on the basis of residual index. Furthermore, the proposed method was applied to quantitatively evaluate impact of grid-connected UPFC on sub-synchronous stability for real power system, and resonant parameter domain of UPFC was worked out. Nonlinear time-domain simulation verified correctness of above theoretical analysis.