Due to its good reliability, direct-driven wind turbines (WTs) are increasing used in new wind farms, especially in offshore wind farms. With the increasing use of direct-driven WTs, a new type of sub-synchronous oscillation problems related to direct-driven WTs has occurred when connected to weak AC grid. Several studies have tried to analyze these oscillation problems, but most of them used the impedance method from the circuit equivalent perspective. Actually, the circuit equivalent and mechanical equivalent can be transformed to each other. To understand the sub-synchronous oscillation problems related with direct-driven WTs from the perspective of mechanical equivalent, this paper verifies that the dynamic interactions between the DC-link voltage control loop and the phase-locked loop (PLL) are the same as the two-masses shaft model based on the motion equation method, and proposes a quantitative method to evaluate direct-driven WT’s intrinsic mode characteristics in sub-synchronous range. Results are all verified in MATLAB/Simulink. In addition to impedance method, this paper provides another modal perspective to understand sub-synchronous oscillation problems related to direct-driven WTs.