Abstract: This paper introduced the characteristics of underbalanced drilling and operation cautions. Detailed analysis and comparison were made for operation processes of well Bai 21 with HP and low permeabillty and well Mao 8 with LP and leakage. The underbalanced drilling technology(UDT) is complicated, and it is a high-technical content and operation risk technology. But UDT avoided the damage to payzone during drilling. By reducing the pressure of borehole liquid column, the undercompaction was resulted in. The penetration rate can be increased by 2~6 times and the bit life can be extended by 2~3 times.The occurrence of differential sticking and lost circulation was eliminated.