Double-sided argon arc welded joint of a titanium ally, which is named as TA15, was treated by laser shock peening (LSP). The hardness, residual stress, fatigue life and the mechanism of fatigue crack nucleation of the joints before and after LSP strengthening were compared and analyzed. The results show that: the surface hardness of the welded joint was increased; the initiation location of fatigue crack was transferred from the surface defect with high stress concentration to the interior. At the stress ratio of 0.1, the high stress level (590 MPa) life was improved by about 4.6 times, and the medium stress level (330 MPa) life was improved by 9.9 times. Murakami model as well as modified Murakami formula was used to evaluate and calculate the fatigue strength of double-side welded TA15 procedures by laser shock peening. The maximum error between the calculated results and the experimental results is 22.6%, and the minimum error is 7.78%.