The multipactor phenomenon between metal wall and dielectric window disk of pill-box window can behave as double surface multipactor, which is affected by the normal electric field of TM11 mode. The Monte Carlo code is used to build up simulation model, calculate the multipactor susceptive curve, threshold voltage, evolution of particle number, and the trajectory of particle motion under the action of double surface normal field between alumina window and copper wall. Through investigating the behavior of secondary electrons, the regularity of normal field double surface resonant multipactor and non-resonant multiapctor is achieved. Besides, the feature of the transform from double-surface multipactor to single-surface multipactor is also obtained. This research can provide a theoretical basis for window breakdown mechanism analysis.