  Within general partial-wave mixing, a method for reducing the high dimension
of the finite-volume Hamiltonian from Hamiltonian effective field theory is
proposed. This method provides a new viewpoint on partial-wave mixing, and a
set of matrices that can reflect the degree of partial-wave mixing. An example
of isospin-2 $\pi\pi$ scattering is used to examine the consistency between
this method and L\"{u}scher's method.
