A self-injection locked continuous-wave (CW) single-frequency tunable Ti:sapphire laser is demonstrated in this paper. Unidirectional operation of the presented Ti:sapphire laser is achieved by using a retro-reflecting device which can retro-reflect a seed laser beam from one direction back into the counter-propagating field. On the basis, the influence of the transmission of output coupler on the unidirectional operation is investigated and it is found that stable unidirectional and single-frequency operation of the Ti:sapphire laser is achieved when the loss difference between both output directions is larger than a certain value, which is easy to be realized by choosing the transmission of output coupler. When the output coupler with transmission of 6.5% is utilized, the maximal 5 W CW single-frequency Ti:sapphire laser with stable unidirectional operation is obtained with the pump power of 18 W. The measured power stability and M2 are better than ±0.9% and 1.1, respectively. The maximal tuning range and continuous frequency-tuning ability are 120 nm and 40.75 GHz, respectively.