A device for controlling fault ride-through of a power grid of a flexible direct-current transmission system comprises a control unit, a detection unit, a switchable braking resistor unit, a power voltage regulation unit and a full-bridge braking unit, wherein the switchable braking resistor unit is used for primary power release of output of a rectification end, the power voltage regulation unit is used for secondary power release of output of the switchable braking resistor unit, and the full-bridge braking unit is used for tertiary power release of output of an inversion end. By a method for controlling fault ride-through of the power grid of the flexible direct-current transmission system, direct-current side busbar voltage and alternating-current side busbar voltage are detected in the real time, and when a fault signal is detected, a direct-current change-over switch is switched off, and a second fully-controlled switch is switched on to dynamically enable a first fully-controlled switch to be on for dynamic voltage regulation, so that maximum residual power transmission is achieved. The method is capable of effectively inhibiting overvoltage of a direct-current busbar, avoiding latching of devices of the direct-current transmission system and guaranteeing maximum residual power transmission, and the devices of the direct-current transmission system can be normally operated during fault.