Abstract: In order to explore the applicability of“Effective Radius”theory to the drain pipe wrapped in synthetic envelope,this paper conducts field pipe drainage experiments,which consider the types and the thickness of the envelope. The drainage processes are monitored and compared with those computed by the drainage theory formula and the numerical simulation of groundwater movement combined with the theory of“effective radius”. It is found that the effective radius of drainage is mainly influenced by the permeability coefficient and the thickness of envelope,which results in a bigger radius with a higher thickness. This is contrary to the field test results and the calculation results of the drainage theory formula. Therefore the traditional theory of effective radius of outsourcing material drainage is not suitable for synthetic outsourcing materials. It is necessary to revise the theory for better estimating the discharge of pipe drainage.