As a 6000 m rated class autonomous underwater vehicle (AUV), the “Qianlong I”, using for deep sea manganese nodules detection, was launched in China in 2011 and functioned as one of the key projects in the Twelfth Five-Year Plan period. Owing to its large diving depth, it usually needs 2-3 times of diving to complete navigation balancing in destined depth when navigating in unknown deep ocean. A single direction variable buoyancy system was developed for the “Qianlong I” AUV in this paper. The system was fixed outside the vehicle when diving in unknown waters for the first time and automatic buoyancy adjusting was implemented at the destined navigation depth. Through buoyancy adjusting, the optimum balancing was achieved at the destined diving depth in the first trial and detecting tasks could be carried out synchronously in such a depth. There is no need to dive for balancing in deep sea detection in unknown waters as a result of the variable buoyancy system, substantially improving the economical efficiency and convenience of the “Qianlong I” utilization. The system has the merits of independence functions, small size and easy access to use and maintenance. The work introduced in this paper has high engineering value and some theoretical significance for deep ocean vehicles.