The invention relates to the technical field of electric power system voltage automatic control, in particular to a thermal power plant frequency converter low voltage ride through device with alternating-current input backup and direct-current input backup. The low voltage ride through device is arranged between a power grid power distribution cabinet and a frequency converter, and comprises a first execution unit, a second execution unit, a storage battery set, a rectifying circuit, a boosted circuit, an output execution circuit and a signal regulating board. When voltages drop, the signal regulating board controls an alternating-current power supply to be rectified through the first execution circuit and the rectifying circuit, or after the storage battery set outputs direct-current voltages through the second execution unit, pressure boosting is carried out through the boosted circuit, and a power grid is compensated through the output execution circuit. The low voltage ride through device is safe, reliable, high in sensitivity and capable of guaranteeing the continuous and stable operation of a key auxiliary machine and a thermal power generating unit.