The theory on small intrusions forming large deposits put forward by academician Zhongli Tang was a recapitulative summary by Long-term studying of magmatic Ni-Cu sulfide deposits which related to mafic-ultramafic intrusions. And this theory got a common response by exploration industry in the world. Many mineral exploration practices have effectively illustrated that the hypabyssal and small intrusion could always form the larger ore deposits. Our study is about the mineralization of Ni-Cu sulfide deposits which related to mafic-ultramafic intrusions in the northwestern of China. In this paper，we have discussed the theory of small intrusions forming large deposits，considering that mantle magma deep part liquation and injection is the main mechanism of this theory. And another theory of magma conduit systems，may cause the local enrichment of mineral，and like that theory of small intrusions forming large deposits. After summarizes the prospecting fact of magmatic Ni-Cu sulfide deposits in the northwestern of China，we expand the scientific connotation of the theory on small intrusions forming large deposits，and make it more realistic meaningful in guiding prospecting.