The rotation vector method is a method in which the vector length is constant and the vector is represented by angular velocity rotation. Complex number representation is a representation method in which two orthogonal components are equivalent to rotation vectors. In practical simple harmonic vibration, the motion law of particles is sine (or cosine) law. The rotation vector can be decomposed into two orthogonal components: the real part and the imaginary part. Assume that the initial phase angle of the rotation vector is 0 degrees. Then the real part after decomposition is cosine and the imaginary part is sine. These two parts (the real part and the imaginary part form a complex plane, and this representation is a complex representation), so the rotation vector is equivalent to the complex representation.