The electron correlation[1] plays a key role in understanding of the low temperature properties of quantum many body systems with low electron density,especially for low-dimensional systems,in which the electron correlation is strong.It is well known that the electron correlation is a quantum many body effect,and it depends upon not only the Coulomb interaction of electrons but also the effective dimensions of electrons.Consequently,it is difficult to unambiguously define the electron correlation as some "parameters" that can be perturbatively treated like that in the usual many body perturbation theory.[2,3] Although a lot of efforts have been made to a variety of theoretical methods,it is still unclear how to effectively describe the electron correlation in quantum many body systems,except some one-dimensional systems and single impurity models which can be treated exactly by some special methods,such as the Bethe ansatz[4] and bosonization method.