To address the problem of sub-synchronous oscillations induced by the weak AC grid of direct-drive wind farms, this paper proposes a fixed-time sliding mode control to achieve sub-synchronous oscillation suppression. Firstly, according to the disturbance equation of the weak AC grid of a direct drive wind farm, the mechanism of sub-synchronous oscillations is studied, and the characteristics of sub-synchronous oscillations are analyzed under different strengths of weak AC grids. Furthermore, the dynamic equation of the grid-side converter current tracking error is established, and the hyperbolic tangent function is used instead of the sign function in the sliding variable to tackle the singularity problem encountered in the traditional fixed-time terminal sliding mode control. Finally, a fixed-time non-singularity terminal sliding mode control method is proposed, in which the current error can be driven to a neighborhood around zero in the fixed time. The theoretical analysis is conducted to prove the fixed-time stability of a closed-loop system via the Lyapunov theory. Comparative numerical simulation is carried out to demonstrate that the sub-synchronous oscillations of the direct-drive wind farms can be suppressed in a fixed time under different strengths of weak AC grid working scenarios.