Abstract: Isothermal annealing heat treatment experiment of C-Mn cold-rolled sheet with carbon content 0.159 1wt% was carried out by using thermomechanical simulation testing machine at different temperatures and holding time, and then the steel's tensile properties was tested. A segmented tensile properties mathematical model based on the weighting method was built and verified. The results show yield strength of the tested steel decreases with the increase of isothermal temperature and holding time. The change rules of tensile strength with isothermal temperature and holding time are relatively complex; from a general view, the tensile strength decreases with the increase of isothermal temperature and the holding time. The change trend of elongation is opposite to that of the strengths. The segmented mathmatical model has good accuracy, the relative errors of yield strength, tensile strength and elongation from calculation and experiment are 3.93%, 3.21% and 19.91%, respectively.