The invention discloses a method and system for starting a protective action after a fault on a converter transformer valve-side area, and belongs to the technical field of relay protection. The method of the invention includes: starting transformer differential protection; obtaining currents on both sides of a converter transformer valve-side area, and determining a differential current and a braking current according to the currents on both sides of the converter transformer valve-side area; determining an operating point according to the differential current and the braking current when thedifferential current is greater than a protective action starting value; if the operating point is located in a ratio braking characteristic action area, determining the second harmonic content of the differential current after one cycle; when the second harmonic content is less than a preset value, starting the action protection; when the second harmonic content is greater than the preset value,if the polarity of a DC component in the differential current is positive after the fault, staring the protection action. The method is used when a single-phase ground fault occurs on a rectifier-side converter transformer valve side, and the converter transformer differential protection can reliably respond, is started in case of an intra-area fault, and is blocked in case of an extra-area fault.