To obtain accurate residual stresses in linear friction welding of TC17 alloy, a numerical relationship between the elastic constants and phase fraction of TC17 alloy was established. The elastic constants with gradient distribution were obtained by combining the phase fraction and introduced into the contour method. The results show that the elastic constants of TC17 alloy are positively correlated with α phase fraction and present a significant gradient distribution in the welding area with a width of about 4 mm. The relative error in calculating the welding residual stresses can be up to 36.06% if the gradient distribution of phase fraction is ignored. The corrected residual stresses in the welding area exhibit a bimodal distribution, with a peak stress of ~442 MPa located at the edge of the heat-affected zone. The results demonstrate the necessity of considering the phase fraction with gradient distribution.