  In this paper, I have proved that for a class of polynomial differential
systems of degree n+1 ( where n is an arbitrary positive integer) the
composition conjecture is true. I give the sufficient and necessary conditions
for these differential systems to have a center at origin point by using a
different method from the previous references. By this I can obtain all the
focal values of these systems for an arbitrary n and their expressions are
succinct and beautiful. I believe that the idea and method of this article can
be used to solve the center-focus problem of more high-order polynomial
differential systems.
