  In this paper, We find that the angular distribution of two-body decay and
scattering process that can be studied by introducing the helicity method.It
has been argued that the angular distribution only depends on magnitude of the
decaying amplitude which final particle momentum is along the $Z$ axis.
Therefore, when calculating the angular distribution,we only need to know the
helicity amplitude in the $Z$ direction. At the same time, we also discuss the
symmetry of physical processes, such as parity and time inversion
transformation and so on. The effects of these symmetric transformations on the
helicity state and the canonical states are studied in this paper. When it is
applied to two body decay process, the symmetry of the amplitude can be
obtained by helicity method. In addition, we also introduce the density matrix
to study the polarization of the final state particles, and discuss some
inherent symmetries of the density matrix. When we study the polarization and
angular distribution of final state particles, we can use these properties to
simplify density matrix without knowing all the elements of the density
matrix.Then we find that there is a close relationship between the partial
wave\cite{2003Tree}coupling constants and the angular distribution parameters,
indicating that these coupling constants can be obtained by experimental
fitting to data.In a word, the properties of two body decay and two body
cascade decay process can be analyzed by helicity method, which has a very good
application .
