It is critical to reveal the evolution rules of two types of α platelets (secondary lamellar α and thin α lath in transformed β matrix) in the tri-modal microstructure of TA15 alloy so as to obtain exceptional properties through microstructural design. The combined effects of isothermal compression (at 950, 965, and 975 °C) and subsequent double heat treatments (a high-temperature heat treatment at 10 °C to 40 °C below the compression temperature followed by a 810 °C heat treatment) on the evolution of the two types of α platelets were studied. The results show that the isolated lamellar α and the thin α lath can be produced by first heat treatment at 10 °C below the compression temperature and the thin α lath in this structure has a colony morphology under air cooling and an acicular morphology under water quenching. Colony and basketweave lamellar α and thin α lath can be produced when the first heat treatment is at 25 °C below the compression temperature. Only the colony and basketweave lamellar α can be produced when the first heat treatment temperature is 40 °C lower than the compression temperature. Besides, the morphology of lamellar α is not substantially influenced by cooling modes after the first heat treatment at the same temperatures.