Abstract: Drainage and drying processing for subsea gas pipeline before commissioning can inhibit the operational of hydrate in the production process or prevent subsea gas pipeline corrosion. Based on the drainage, drying and inerting characteristics and risks of large-diameter, long-distance Liwan 3-1 Gasfield Deepwater Subsea Gas Pipeline, the design of dehydration and drying "train" involved in the dehydration and drying process, design of pigs, analysis and estimate of driving pressure, equipment selection and other technical problems are summarized and analyzed, and the drainage, drying and nitrogen inerting process is designed and implemented. Results show that the dehydration and drying of subsea pipeline connecting Liwan 3-1 Gasfield offshore central platform and onshore gas processing terminal can be completed by the pigging "train" driven by dry air. And the drying technology combining the chemical drying technology using TEG as desiccant with the dry air drying technology is the best choice for the dry processing of Liwan 3-1 Subsea Pipeline, which can achieve the simultaneous implementation of dehydration and drying for large-diameter, long-distance deep water subsea pipelines.