The invention discloses a method for measuring a vertex angle offset of an equilateral triangular prism. The method comprises: step one, two reflectors are fixed to form a 60-dgree angle mirror and the angular bisector of the 60-dgree angle mirror is parallel to the optical axis of emergent light of an interferometer, and an included angle between two beams of light returning in an auto-collimation manner is measured; step two, a to-be-measured equilateral triangular prism is placed between the interferometer and the angle mirror and a to-be-measured vertex angle faces the interferometer directly; and an included angle between two beams of light that passes through two lateral surfaces of the to-be-measured vertex angle of the equilateral triangular prism and then is reflected by two surfaces of the angle mirror is measured by an interference pattern; and then the measurement results are integrated to obtain a vertex angle offset of the to-be-measured equilateral triangular prism. Themethod has advantages of a few of steps, simple operation, and high precision and is used for high-precision measurement of the vertex angle offset of the equilateral triangular prism.