The titanium alloy leading edge protection cap is a critical component of large composite blades, characterized by a complex thin-walled V-shaped long slot structure with deep, narrow, and low-stiffness features. Both its inner cavity and outer profile require shaping through multi-axis precision machining. However, the unbalanced distribution of residual stress introduced by the machining process between the inner cavity and outer profile can induce significant bending-torsion coupling deformation, leading to out-of-tolerance deviations in key dimensions such as the contour accuracy. Therefore, a residual stress deformation control method for the protection cap is proposed based on the optimization of the cutter axis vector in machining outer profile. By optimizing the cutter axis vector in multi-axis machining, this method actively regulates the residual stress distribution on the outer profile, balancing it with that in the inner cavity to achieve precise control of the bending-torsion deformation. Firstly, the hyperbolic tangent function is employed to characterize the residual stress gradient distribution induced by multi-axis machining of TC4 titanium alloy. Subsequently, a finite element simulation model for the residual stress-induced deformation of the protection cap is established on the Abaqus software platform, where the residual stress is applied as loads using the thin-shell stress-fitting method. Based on this model, a quantitative mapping relationship between the cutter tilt angle and the deformation at key locations is established, thereby completing the optimization of the cutter axis vector for outer profile machining. Finally, the effectiveness of the proposed method is validated through machining experiments on multiple parts. Experimental results show that the contour error of the optimized parts is reduced by an average of 23.41%. This method provides a reliable technical solution for the high-precision multi-axis machining of titanium alloy leading edge protection caps, holding direct engineering significance for enhancing the manufacturing quality and service reliability of composite blades.