Bio-based PA5T/56 was successfully prepared by a nover modulated polymerization on this condition when the relationship between temperature and pressure was strictly controlled during the process to render the reactivity ratio of each monomers basically the same. In this case, the obtained PA5T/56, with approximately alternating copolymerization structure, possesses better physical and chemical performance and melt flowing properties. Meanwhile, the real-time sampling and testing was operated during the process to get the experimental values of reactivity ratio. Furthermore, the chain growth process of the ternary polymerization reaction was also calculated and simulated by referring to the Mayo-Lewis formula as well as using the Monte Carlo method, and a probabilistic statistical treatment for estimating the reactivity ratio was given. Finally, by comparing the results, it can be found that the experimental values of the reactivity ratio were in general accord in the calculated values with reference to the Mayo-Lewis formula and the simulated values of the mathematical model, the values of \({r}_{12}\) and \({r}_{13}\) are basically the same, which confirmed the successful synthesis of the bio-based PA5T/56 with approximately alternating copolymerization structure, and that the established mathematical model for estimating the reactivity ratio is relatively accurate and is applicable to the ternary polymerization.