A simple model has been successfully developed to explain the incorporation behavior of As and P in growing GaxIn1-xAsyP1-y quatemary alloys on (100) InP using gas source molecular beam epitaxy (GSMBE). In this model there is only one incorporation parameter, i.e. the As to P incorporation ratio, which was determined by fitting the calculated solid composition ratio of P/As to the experimental results. It is found that when x is fixed at 0.263 the incorporation ratio is about 16 at a growth temperature of 490 degrees C and slightly increases when the growth temperature decreases. Additionally, the RHEED pattern observations and the growth temperature dependence are also discussed. This model can be expanded to the whole range of compositions by adjusting only one fitting parameter, k, and can provide a very useful guide for the epitaxial growth of GaxIn1-xAsyP1-y quatemary alloys on InP substrate by GSMBE.