First, the existing field loss protection of a synchronous condenser is analyzed. The results show that the existing protection cannot reliably act in the case of a partial field loss fault of the condenser in the low voltage condition of the system, and the protection has a dead zone. By studying the variation law of the excitation current after the loss of excitation of the synchronous condenser, it is concluded that the air gap synthetic magnetic potential contains high-order harmonic components after the loss of excitation fault of the condenser, and a corresponding harmonic current will be generated in the stator winding. Thus a method of excitation loss protection of the condenser based on harmonic detection is proposed. A new excitation loss protection strategy is constructed using the even-order harmonic current content in the stator winding. This solves the problem that the loss of excitation protection cannot identify partial loss of excitation faults in low voltage conditions of the system. Finally, a Zhalute-Qingzhou ±800 kV UHVDC transmission model with a synchronous condenser is established in PSCAD/EMTDC, and the simulation results verify the feasibility of the scheme.