This paper introduces the performance parameters of the CYS-300 full hydraulic geothermal well drilling rig, the design of key mechanical components and the design of the hydraulic system. The rig selects the gear pump as the power component of the hydraulic system, and cooperates with the hydraulic multi-way valve to realize the confluence of the hydraulic oil of the power head dual hydraulic motor and the actuator for the intake cylinder, and realizes the series-parallel operation of the power head double hydraulic motor through the hydraulic control two-speed valve. The output of four kinds of speed and torque can simplify the design of the hydraulic system, reduce the energy loss of the hydraulic system, and reduce the maintenance and use cost while meeting the design requirements of the drilling machine performance parameters. The drilling rig adopts the three-chain double-speed mechanism of the oil cylinder to realize the feeding and lifting of the power head, and has the advantages of simple structure, reliable performance, strong impact resistance and stable lifting. The application of on-site engineering shows that the CYS-300 full hydraulic geothermal well drilling rig has stable and reliable performance, strong application ability in process and complex stratum, safe and comfortable operation and high drilling efficiency.