The external Q (Q_(ext)) of the dipole mode is a key parameter of the Cavity Beam Position Monitor (CBPM).It determines the amplitude and length of the dipole mode signal.In this paper,Q_(ext) of a CBPM whose waveguides were open to the air was simulated and measured,and the results agreed with each other.Then four waveguide-to-coaxial cable adpaters were adjusted and assembled to the CBPM,and Q_(ext) remained unchanged.This progress provides a reliable method to evaluate Q_(ext) in the physics design without simulating the structurally complex adapters.