Being a widely used design method and theory, SAPB plays an important role, especially in the process of the overall design of the product. In this paper, this method is used to study the overall scheme design of radiation well horizontal drilling rig. First of all, from the analysis on the general function of the horizontal drilling rig, abstract the drilling machine function, then decompose the total function with analysis tools of SAPB design method to establish functional decomposition tree and function structure diagram of the horizontal drilling rig, finally the rig overall design scheme is got. This research can provide the convenience for the further design optimization of the drilling machine.