The transformation-induced plasticity (TRIP) seamless steel tube, which is expected to be used in the hydroforming process, was prepared with piercing, cold-drawing and two-stage heat treatment process (intercritical annealing, “IA” and isothermal bainite treatment, “IBT”). The current study focused on the effect of IA holding time on microstructure and mechanical properties of TRIP seamless steel tube at a predetermined other heat treatment conditions, to maximize the volume fraction and stability of retained austenite as well as to obtain a TRIP seamless steel tube with good combination of strength and ductility. The microstructure and retained austenite volume fraction of the heat-treated steel tubes were studied via optical microscopy, transmission electron microscopy and X-ray diffraction. Two-stage heat treatment carried out in an electrical resistance furnace and a salt-bath furnace showed that for a predetermined IA temperature, IBT temperature and time, the content of retained austenite as well as total elongation increased first and then decreased with increasing IA time in the set time range in this study. When IA time was 10 min, the largest retained austenite volume fractions for both steel tubes was obtained. As a result, the TRIP seamless steel tube with an austenite volume fraction of 7.02%, total elongation of 35.5% and ultimate tensile strength of 618 MPa was successfully obtained.