  As a simple but important example of dynamical black hole, we analysis the
dynamical black hole in $n$-dimensional Vaidya spacetime in detail. We
investigated the thermodynamics of field equation in $n$-dimensional Vaidya
spacetime. The unified first law was derived in terms of the methods proposed
by Sean A Hayward. The first law of dynamical black hole was obtained by
projecting the unified first law along the trapping horizon. At last, the
second law of dynamical black hole is also discussed.
