Hard-plastic mudstone is usually encountered in deep geothermal drilling， which seriously affects the drilling efficiency. Based on the example of deep geothermal drilling project in Xiong^，an New Area，this paper deeply analyzes the reasons of low mechanical drilling rate in hard plastic mudstone， studies and forms the scheme of optimized and improved bit. The practical application proves that the mechanical drilling rate in hard plastic mudstone formation can be effectively improved by using the personalized PDC bit according to the lithology characteristics of the formation combined with the screw drilling tool. The impact resistance of PDC bit can be effectively improved by using special-shaped compacts such as triangular prismatic and triangular and the service life in gravelly formations can thereupon be increased， in addition， the mechanical drilling rate in hard plastic mudstone formation can also be increased. At last， a drill bit application scheme suitable for hard plastic mudstone formation in deep geothermal drilling is made.