A continuous annealing process is used to treat transformation-induced plasticity (TRIP) steel containing carbon, manganese, and aluminum and obtain the steel-plate samples at different temperatures and holding time of isothermal bainite transformation (IBT). Further, the microstructure, element distribution, and retained austenite of the steel treated by different processes have been characterized using scanning electron microscopy, electron probe microanalysis, electron backscattered diffraction, transmission electron microscopy, and X-ray diffraction. The stability of retained austenite under different bainite isothermal conditions and its effects on the plasticity and work hardening of experimental steel are studied. The results denote that the microstructure of experimental steel treated by different processes comprises ferrite, bainite, retained austenite, and small amounts of martensite. With increasing the temperature of IBT from 380 ℃ to 420 ℃, the volume fraction of retained austenite, mass fraction of carbon in retained austenite, and product of tensile strength and elongation increase gradually. As the temperature of IBT further increasing up to 460 ℃, those three parameters decrease gradually. At the IBT temperature of 420 ℃, the stability of retained austenite as well as the comprehensive mechanical properties are observed to be optimal. When the time for holding the temperature of IBT at 420 ℃ is 180 s, the corresponding volume fraction of retained austenite is 10.7%, mass fraction of carbon in retained austenite is 1.069%, yield strength is 455 MPa, tensile strength is 681 MPa, total elongation is 31.7%, and the product of tensile strength and elongation is 21.59 GPa·%. The extension of holding time of IBT can accelerate the bainite transformation, improve the volume fraction and stability of the retained austenite, and enhance the comprehensive mechanical properties of TRIP steel. It is necessary to ensure sufficient volume fraction of the retained austenite to induce the TRIP effect, ensure proper stability during continuous work hardening, and improve the plasticity.