  With proposing a unified description of the fields variation at the level of
generating functional, we obtain a new identity for the quark-gluon interaction
vertex based on gauge symmetry, which is similar to the Slavnov-Taylor
Identities(STIs) based on the Becchi-Rouet-Stora-Tyutin transformation. With
these identities, we find that in Landau gauge, the dressing function of the
ghost propagator approaches to a constant as its momentum goes to zero, which
provides a strong constraint on the infrared behaviour of ghost propagator.
