The neutral particle analyzer(NPA) is one of the crucial diagnostic devices in a Tokamak facility. The stripping unit is one of the main parts of the NPA. A windowless gas-stripping room with two differential pipes has been constructed in a parallel electric and magnetic fields(E//B)NPA. The pressure distributions in the stripping chamber are simulated by ANSYS Fluent together with MolFlow+.Based on the pressure distributions obtained from the simulation, the stripping efficiency of the E//B NPA is studied using GEANT4. Hadron reaction physics is modified to track the charge state of each particle in a cross-section-based method in GEANT4. The transmission rates(R) and stripping efficiencies f_(+1) are examined for particle energies ranging from 20 to 200 keV with the input pressure(P_0),ranging from 20 to 400 Pa. According to the combined global efficiency, R ×f_(+1), P_0= 240 Pa is obtained as the optimum pressure for the maximum global efficiency in the incident energy range investigated.