Abstract: SAP (sustained annular pressure) was observed in a exploration well in Persian Gulf. And during its abandonment, perforation and cementing in the gas source layers was tried many times to kill the SAP, but no success is ever achieved. In this paper, the dual casing (Ø244.475 mm and Ø339.725 mm) in the upper part of gas source layers was milled, and gas block cement plug was set in the milled section. It is verifed by negative pressure test that cement plug can isolate the gas source layers effectively. Field application shows that multi-layer casing milling and cement plug technology can solve completely the SAP problem which cannot be dealt with by the conventional perforation and cementing technology.