The traditional method of describing precession nutation is spherical trigonometry. The defect of spherical trigonometry is that it is arbitrary in determining the positive and negative of an edge or Angle. When applied to the derivation of precession and nutation matrix, it is more complicated. In this paper, the coordinate system is established by defining the normal directions of different planes. Nutation matrix is given by vector method. Precession matrix is similar. The first group of nutation angles (nutation in longitude, nutation in obliquity) describes the changes of the instantaneous true equatorial plane with respect to the instantaneous ecliptic plane. The second group of nutation angles describes the change of the instantaneous true equatorial plane with respect to the ecliptic plane of the reference epoch. In this paper, the theoretical relationship between two sets of nutation angles is given, and an error in Aoki et al. (1983) formula is corrected. In fact, the meaning of the second group of nutation Angle is more clear, and it is more suitable for the conversion between the mean equatorial coordinate system of reference epoch and the true equatorial coordinate system of date. The characteristics of the IAU2000A_R06 model are also analyzed.