The quasi-static tensile behavior and mechanism of TA2 pure titanium with three grain sizes of 41 μm, 63 μm, and 323 μm has been studied using OM, SEM, EBSD, etc. The results show that the coarsening of the grains results in a higher uniform deformability of TA2. The reason is that the larger the grain size, the stronger the twinning activity during deformation. The twin boundaries divide the grains and reduce the effective sliding distance and hinder the movement of dislocations, so it has a positive effect on the strain hardening rate of the material. When the grain size increases from 41 μm to about 323 μm, the strain hardening rate curve changes from a continuous decreasing trend to a three-stage change trend of decreasing-increasing-decreasing. The high strain hardening rate induced by twins in the coarse grains is the root cause of TA2"s high uniform deformability.