The B block in Saudi Arabia is a High Temperature and High Pressure (HTHP) low permeability condensate gas reservoir. The early exploration wells show good behavior,but with not very high production rate. Indoor experiments show that the oil lock and the water lock are the main cause of the reservoir damage. In order to deal with this problem during the drilling operation, the underbalance drilling is adopted. In the design of the negative pressure value in the process, the well control safety, the borehole stability, the oil lock damage, the water lock damage and other factors are taken a full account with respect to their influences on the bottom negative pressure value selection under 2_-10MPa while drilling. In the calculation of the hydraulic parameters, the influence of the temperature and the pressure on the drilling fluid density and viscosity is considered, the industry approved Drillbench software is used to calculate the sensitivity of the drilling fluid density, and finally to determine the drilling fluid density, which is in the range of 1.69-1.76g/cm³. The main equipment includes the rotating blowout preventer, the DDV, the automatic choke manifold, the four-phase separator, the data transmission and processing equipment, the ignition line, and the automatic ignition system. With the setting up of DDV, it is not necessary to shut down the well in the process. In the M-0001 well Sarah group, the real drilling process shows that the fluid density is in the range of 1.69-1.71g/cm³, the bottom hole negative pressure value in the range of 2.08-3.12MPa. According to the M-0001 well test data, the main reservoir skin factor S=3.44, indicating that MTLH-0001 well drilling fluid has a good effect on the gas reservoirs, and the drilling process will not cause the formation pollution.