Dynamic recrystallization (DRX) is considered as one of the most important microstructural evolution mechanisms to obtain fine metallurgical structures, eliminate defects and improve mechanical properties of products. Although the DRX kinetics models proposed by researchers have some differences in parameters and forms, they are all based on the Avrami function describing the relationship between dynamically recrystallized volume fraction and strain or time. Avrami equation is in the form of exponential function and the kinetics curve of DRX exhibits different when the exponent is assumed to be different (between 1 and 2). Under these conditions, however, the exponential function cannot exactly describe the "slow–rapid–slow"property of the development speed of DRX process. By introducing the velocity of development of DRX process, which is referred to as the variation of the dynamically recrystallized volume fraction with strain, namely, the first partial derivative of the dynamically recrystallized volume fraction to strain, a new kinetics model of DRX was proposed in comparison with the classical kinetics model of DRX. The new model represents the characteristics of DRX: the dynamically recrystallized volume fraction equals zero when the strain is smaller than the critical strain, and the maximum of the dynamically recrystallized volume fraction equals 1; once the strain exceeds the critical strain, the dynamically recrystallized volume fraction slowly increases first, and then rapidly increases, at last slowly increases. Consequently, the new kinetics model is in agreement with the development law of DRX process and includes few parameters which have clear physical meaning and are easy to determine. By conducting Gleeble–1500 thermomechanical simulation compression tests at the temperatures ranging from 523 to 673 K and at the strain rates 0.001, 0.01, 0.1 and 1 s⁻¹, the kinetics model for Mg alloy AZ31B characterized by DRX for instance was built and parameters were determined. Microscopic examination shows that the experimental results are in good agreement with the predicted values, which validates the accuracy of the new kinetics model.Then combined with grain size of DRX model, the kinetic model built under steady state conditions was rewritten as superimposed step form to apply in the prediction of grain size under unsteady state conditions. The simulated data accord with the experimental results by means of quantitative metallography, which verified the rationality of the superimposed prediction method.