The idea that most of the lager deposits were formed by smaller intrusion was firstly proposed on the basis of much prospecting practice in 1979. Through a long term of theoretical researches and guiding prospections， many large and super large deposits have been discovered， so the thought was accepted by more and more scholars. By summarizing the conception， connotation and category of the theory for large deposits formed by smaller intrusion， this paper pointed out that the five key features of this theory are “small， wide， large， high and shallow (potential)”. Through many studies on the ore genesis， we developed the idea of the large deposit formed by smaller intrusion into a theory， and presented metallogenic models， metallogenic mechanisms and metallogenic types for two kinds of large deposit formed by smaller intrusion. One kind is the deposit formed by mafic magma which has experienced deep segregation (preconcentration) at first， then emplacement by pulses and finally concentration into a deposit in the terminal of magma chamber； the other kind is the deposit formed by intermediate-acidic magma， which is presented as mineralization at the head of intrusion (mineralization at the head of gas， liquid， mineral， fluid)， and mineralization in the front of intrusion and lateral space.