In the test of prestressed rock bolt in construction of Jinping Hydropower Station, the actual tensioning length could not meet the requirements. Free tensioning test on the free rock bolt section is carried out in the workshop, and construction tensioning test is carried out in the site, so as to obtain the variation rules of the tensioning length of prestressed rock bolt. The tests showed that the variation rules of the tensioning length of the prestressed rock bolt are complicated under different conditions, and can be affected by construction technologies, rock bolt material, temperature and tensioning force. The tests proved that the tensioning lengths of the rock bolts tested can not meet the theoretical requirements, but the locking capacity of the rock bolts is almost unaffected. Therefore, the control of the tensioning force of the prestressed rock bolts is the key in the control of the tensioning quality of the rock bolts, and the tensioning length can be regarded as the reference value.