Abstract: This paper studied tunnel core drill rig pressure-reduced drilling. In the comparative analysis of the advantages and disadvantages of throttling pressure-reduced drilling loop and overflow pressure-reduced drilling loop, constant pressure variable displacement pump controlling pressure-reduced drilling loop is proposed in this paper. Combining ZDY-1000G pressure-reduced drilling loop, using AMEsim software to simulate the pressure flow characteristics of different circuits, it is concluded that the constant pressure variable displacement pump controlling pressure reducing drilling circuit has the advantages of convenient operation, high safety and strong load adaptability.