Under the assumption that one incident As(4)molecule produces one active As(2)molecule before incorporating into epitaxial alloys, a novel thermodynamic model is developed to describe the molecular beam epitaxy (MBE) growth of III-V compound semiconductors with two group V elements using As-4 flux. A parameter epsilon (d) representing the activation energy of As-4 dissociation process is introduced in the model. High quality InAsyP1-y materials were grown on InP substrates using our home made all solid source MBE (SSMBE) system with valved cracker cells under different substrate temperatures and arsenic fluxes with different As-2/As-4 ratios. By taking into account the As-2/As-4 ratio, the model is used to calculate the compositions of the grown samples. The calculated results are in good agreement with the experimental data. Both the experiments and the calculations show the lower incorporation efficiency of As-4 than that of As-2.