Right dihedral-angle error is key in cube blocks such as corner cube retroreflectors in laser ranging and cube test mass in space-borne gravitational reference sensors. A high-precision comparison method of dihedral-angle using optical differential wavefront sensing is proposed. The principle is to use an optical interferometer to measure the relative wavefront signal of two light beams reflected from two surfaces containing the angle. The relative dihedral angle error of the two samples can be obtained by comparing the two dihedral wavefront differences. Compared with conventional optical methods such as the Fizeau interferometer and autocollimation, this method has the advantages of low cost, simple data processing, compactness, and high accuracy. An interferometer for relative right dihedral-angle error measurement is built and tested. The experimental results show that an accuracy below ±6 μrad within a dynamic range of ±100 μrad has been achieved. Combined with a dihedral-angle standard sample, this method can be widely used for dihedral-angle comparison and calibration.