The decomposition of metastable β phase and microstructural evolution of a forged Ti-22Al-25Nb orthorhombic alloys were investigated by the electron microscopes and phase identification. For the near-Ti2AlNb alloy, the processing temperatures were super-transus (over the transition temperature of the β phase). β phase was ordered before its decomposition and cooling rate could strongly influence the precipitation of the explored Ti-22Al-25Nb alloy. Water quenching could prevent the transformation related to diffusion and air cooling hindered the occurrence of fine lenticular O phase in the matrix. Comparison with previous researches and selected area diffraction patterns revealed that there did exist a displacive precursor that provided nucleation sites for the α2 phase during B2 → α2 transition. Due to the transient structure and diffusive gradient of Nb component during the transformation, a “cellular structure” was assembled and quantities of equiaxed α2 precipitates were acquired. Simultaneously allotriomorph α2 precipitates inclined to grow into coarse plates on the boundaries, which would have a detrimental effect on the mechanical properties of the alloy. Widmanstätten structure was formed during the (B2+α2+O) field and massive transformation of lenticular O phase occurred in the (B2+O) field.