The expansion behavior and partitioning behavior of alloying elements in α+β→β phase transformation of TC21 titanium alloy during continuous heating were carried out by combining dilatometric method, optical microscopy (OM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The results show that the change in the length of the TC21 alloy during the continuous heating process results from the transformation of α phase with hexagonal close-packed to β phase with body-centered cubic and macro volume variation which originates from redistribution of α-stabilizers and β-stabilizers in the alloy during the continuous dissolution of α phase when the temperature is over β transformation temperature. It is found that the content of Al element increases in the α phase with the dissolution of α phase, but it decreases in the β phase. Additionally, the increasing content of Al element leads to decrease in volume of β phase, which is also main reason for shrinking of the alloy during α+β→β phase transformation. However, Mo, Cr and Nb element diffuse to the α phase during α→α+β phase transformation, so that the composition imbalance was found at the α/β interface.