This study coupled both new systems of syndrome essential factor and formula essential factor with the artificial intelligence technique to analyze all the formulas of the Shang Han Lun (伤寒论) to build an AI model of formula-syndrome essential-factor correspondence. It is developed by self-coding with the software of Microsoft Visual Studio C#.NET to design all the functions that we need. One of the methods called back propagation network (BPN) of the artificial neural network technique was employed to build the AI model. This study employed the book of Shang Han Lun (伤寒论) for the materials to undertake the structural analyses. The mathematical model can then be applied to the practical prediction of prescription. That demonstration shows that it is feasible and practical to forecast the precise formulas for the patients. However, some divergence appears when the amount of neuron nodes is too large. Two methods were employed to improve the efficiency of convergence, i.e., the adjustment of learning rate and the conjugate gradient method.