The role of a beam splitter for an interference infrared spectrometer is presented. According to the requirement that the peak-to-valley (PV) value variation of a beam splitter should be 0.05λ (λ=632.8 nm) after it is fixed, the supporting method of the beam splitter is determined. By analyzing the PV value variation of the beam splitter when it is in the states of flatness, verticality and zero-gravity by a finite element method, it is derived that the PV value variation of the beam splitter can be controlled by applying different pressure to the tablet used. It is found that when the beam splitter is in the state of verticality, its PV value is varied greatly and the maximum pressure which can be applied is 0.866 MPa. The elastic supporting material is selected, its size is determined and a supporting sample is made. The actual measurements show that the analysis by the finite element method can meet the high precision supporting requirement of the beam splitter fully. This provides the basis for the accuracy improvement of the instrument system.