  In quantum field theory, the phase space integration is an essential part in
all theoretical calculations of cross sections and decay widths. It is also
needed for computing the imaginary part of a physical amplitude. A key problem
is to get the phase space formula expressed in terms of any chosen invariant
masses in an $n$-body system. We propose a graphic method to quickly get the
phase space formula of any given invariant masses intuitively for an arbitrary
$n$-body system in general $D$-dimensional spacetime, with the involved momenta
in any reference frame. The method also greatly simplifies the phase space
calculation just as what Feynman diagrams do in calculating scattering
amplitudes.
