  Background treatment is crucial to extract physics from precision
experiments. In this paper, we introduce a novel method to assign each event a
signal probability. This could then be used to weight the event's contribution
to the likelihood during fitting. To illustrate the effect of this method, we
test it with MC samples. The consistence between the constructed background and
the background from MC truth shows that the background subtraction method with
probabilistic event weights is feasible in partial wave analysis at BES III.
