The secondary phases exert strong influence on the mechanical properties, processing properties and performance of steels. The volume fraction, size, shape and distribution of the secondary phases must be effectively controlled in order to obtain the excellent properties and performance. On the basis of equilibrium solubility product formula, precipitation thermodynamics and kinetics, and classical nucleation and growth theory, the theoretical calculation method and procedure for the PTT (precipitation fraction–time–temperature) curve and NrT (nucleation rate–temperature) curve for main secondary phases precipitated in austenite and in ferrite have been proposed. The calculation results are well in agreement with practical experimental results. By analyzing the calculation results, some important deductions for the practical control of secondary phases in steels have been made.