Abstract: A series of breccia was formed during the overlap intrusion in the Laoyachao Pb-Zn-Au polymetallic deposit. The magma crypto-explosive breccia, which is the main host of the deposit, includes crypto-explosive breccia, shatter breccia and fluidized breccia. The Late Triassic-Middle Jurassic karst sedimentary bodies, with high porosity and containing groundwater, are widely spread in the ore field. When the magma intrusion took place, the external environment changes caused the reduction of pressure and temperature, with re-boiling of magma and constant exhalation of fluid. In locally closed environment, the fluid pressure gradually increased. With rich groundwater in the karst rocks, continuous gasification of the groundwater resulted in rapid increase of gas pressure. In the well confined area by the overlap, explosion took place, forming crypto-explosive breccia. In the external zone of the explosion, shatter breccia was formed. The fluidization after the explosion, then, formed the fluidized breccia. Based on the study of the characteristics of the cryptoexplosive breccia in the Laoyachao Pb-Zn-Au polymetallic deposit, with the rock formation, structure and intrusive rocks, the forming process of the crypto-explosive breccia is discussed. The analysis on the relationship between the lead-zinc-gold ore bodies and the crypto-explosive breccia will be helpful for further prospecting in the mining area.