A Sagnac interferometer whose optical diameter is 80mm and spectral channel is 75 is introduced based on its working principle. The method of precision alignment is introduced in detail, which includes the installation of primary alignment plane, the precise location of splitting prism, the precise adjustment of long-arm and short-arm reflector. Through the installation of alignment reference and the masterly reference transformation, the precise alignment of separated Sagnac interferometer is achieved based on the principle of auto-collimation, whose position accuracy is better than 0.01mm, angle accuracy is better than 1", primary working plane is better than 1".