To quantitatively research the microstructural evolution of lamellar α phase in titanium alloys, large size cakes of Ti-17 alloy with initial lamellar microstructure were isothermally-forged at different strains in α+β phase field followed by solid-solution and aging treatment. The changes of α thickness and morphology in different locations (center, midradius, edge) of the cakes deformed to different strain levels were measured and the imposed strain was estimated using finite element analysis. It is found that increasing of forging strains causes an increase in thickness of α phase. The modification of α morphology depends strongly on the forging strains and the locations in the cakes. The distribution curves of α morphology parameter Feret Ratio were characterized by no peak at lower strains, and a single peak appears at Feret Ratio 1.5-2.5 at larger strains. Moreover, the peak values increased with increasing of the height reduction, and the distribution curves at the center have the largest peak values. Increasing of forging strains will cause the high-Feret-Ratio lamellae to decrease but low-Feret-Ratio α to increase noticeably. Sufficiently imposed strains also can improve the microstructure homogeneity in different locations of the cakes. The effect of the effective strains on the percentage of α phase with different morphologies is also quantificationally summarized. The average strains for initiation of globalization are of the order of 0.4 and those for completion are of the order of 1.0, which are lower than that of conventional α+β titanium alloys