Amorphous SiNx films were deposited on p-type Si(100)substrates by magnetron sputtering technology. The samples were then detected by a Bruker Tennsor 27 Fourier transform spectrometer. One intense absorption band of the SiNx films (from 812 to 892 cm(-1)) which was assigned to the stretching vibration mode of Si--N--Si bond was detected by Fourier transform infrared (FTIR) spectroscopy. Obviously, it was showed that a red shift of the absorption peak occurred in the FTIR spectrum with the sputtering power increasing; nevertheless, a blue shift of the absorption peak occurred after annealing with the temperature increasing. In the present paper, the deposition process and inner structures of the SiNx films were studied according to RBM (random bonding model)and CFM (central force model). With the increase in the ratio of N(Si)to N(N), the angle of the Si--N--Si changed and the different structures were formed correspondingly. Therefore the Si--Ny--Si(4-y) (0 < or = y < or = 4) models were set up to explain the inner structure of the SiNx films. The investigation showed that Si--N4 tetrahedron, Si--N--Si3, Si--N2--Si2, Si--N3--Si and Si--Si modes were formed accordingly in the SiNx films with the sputtering power increasing. And five models in total were formed during the deposition process. Different stretching vibration modes of Si--N--Si bond were corresponding to the different inner structures of thin films prepared by different sputtering power. With the temperature increasing, the activity of atoms increased which would let the angle of the Si--N--Si go to identical. As a result, Si3N4 and Si nanocrystals were formed with the phase separation of SiNx films during the annealing process with higher temperature, which would result in a blue shift to 870 cm(-1) (the standard absorption peak of Si3N4).