Detailed analyses and the correlations of coherent perfect absorption (CPA) in single-channel and two-channel structures are performed by the transfer matrix and scattering matrix method. The results show that there are many possibilities for single-channel and two-channel structures to satisfy the occurring of CPA. Moreover, with the same incident wavelength and refractive index of dielectric, the length of dielectric for two-channel CPA is twice that for single-channel. For instance, it is 3.701 m/7.402 m for single-/two-channel CPA as 756 nm wavelength is incident on Si material. Our results facilitate the application of CPA in optical modulator, switches, detector, etc., and they are also of significance for Si-based integration of optical circuits in optical communication and computer.