Abstract: To address the liner cementing issues such as unsure or difficult release due to short liner length,light liner weight and high friction in deep,ultra-deep and highly deviated wells,the holddown sub was developed for liner cementing.The holddown sub was designed with locking and unlocking mechanisms.After the liner hanger was set and positioned,the holddown sub could be hydraulic locked to hold the liner on upper casing and provide downward force for determining the release through a weight indicator.The strength of the holddown sub was verified by finite element analysis and the results met the design requirements.The ground test and field application results showed that the holddown sub had successfully achieve the function of locking and unlocking and each technical index had met the design requirements.Thus,the holddown sub could solve the problem of unsure releasing in liner cementing and difficult release in highly deviated wells.