Abstract: The rapid isolation and ride-through of the DC side faults is required urgently for DC transmission systems based on the modular multilevel converter (MMC). Based on the induction and improvement of the existing sub-modules, this paper proposes a bidirectional switch clamped full bridge sub-module (BCFSM) with DC fault current symmetrical clearing capability. After blocking, the DC fault current is led to charge all capacitors in the bridge arm in series, which ensures both the fast fault current clearing and the excellent capacitor voltage balance in the bridge arm during blocking. Meanwhile, compared the fault ride-through capabilities with other sub-modules, BCFSM performs better in terms of economy because the bidirectional switch has the function of both clamping and bypass channel. Simulation results based on MATLAB/Simulink show that BCFSM-MMC is capable of fast DC fault current clearing and fast restart after fault clearing, which improves the reliability of DC fault ride-through while reducing the cost of converter construction and operation.