To address the problems of asymmetric arm operation and fundamental and second-harmonic circulating currents caused by submodule faults in a modular multilevel converter （MMC） under hot standby in a flexible direct current system，a fault-tolerant control strategy coordinated by an improved PIR circulating current control and an inner-loop current sliding mode control is proposed.By introducing a three-parameter notch filter to precisely separate the circulating current components，the improved PIR control achieves the coordinated suppression of fundamental and second-harmonic harmonics.Simultaneously，the traditional PI inner-loop control is replaced by the sliding mode control to significantly improve the dynamic response speed and robustness of the system.Simulations based on Matlab/Simulink and RT-LAB hardware-in-the-loop experiments show that the proposed strategy can effectively suppress the circulating current harmonic components，rapidly stabilize the capacitor voltage and direct current，and reduce the grid-side alternating current fluctuation.Furthermore，it achieves stable system recovery with superior regulation performance after a submodule fault，verifying its multi-frequency harmonic suppression capability and dynamic control advantages.