The stress-strain curve of an alpha-beta Ti-8Mn alloy was measured and then it was calculated with finite element method (FEM) based on the stress-strain curves of the single alpha and beta phase alloys. By comparing the calculated stress-strain curve with the measured one, it can be seen that they fit each other very well. Thus, the FE model built in this work is effective. According to the above mentioned model, the distributions of stress and strain in the alpha and beta phases were simulated. The results show that the stress gradients exist in both alpha and beta phases, and the distributions of stress are inhomogeneous. The stress inside the phase is generally higher than that near the interface. Meanwhile, the stress in the a phase is lower than that in the beta phase, whereas the strain in the alpha phase is higher than that in the beta phase.