The main reservoir of Longfengshan Gas field in Changling Fault Depression is distributed in Yingcheng and Huoshiling Formation where the igneous rock is abundant. The high quartz content and abrasive nature of the reservoir rock results in low ROP and short footage per bit， which greatly increases the non-productive time such as tripping and bit changing and thereby seriously restricts the drilling speed. In this paper， the rock mechanic section is established and corrected by the well logging data and the rock mechanics experimental study. A 6-blade shaped PDC bit was designed for the volcanic rock formation in the block by PDC bit composite design and optimization. It has been applied in 3 wells，compared with other common PDC bits， the average footage is increased by 42.2% and the average machine drilling speed is increased by 36%， which shows a remarkable effect for drilling speed up and cost reduction.