The work aims to Select sphagnum as the representative plant of the Bryophyta, discuss the feasibility of preparing nanocellulose from Sphagnum, and investigate the application performance of the prepared Sphagnum nanocellulose in the sodium alginate composite films, so as to expand the potential applications of chemical inclusions in bryophytes. The cellulose component in Sphagnum was extracted by soda pulping, and further transformed into nanocellulose by sulfuric acid hydrolysis. The prepared Sphagnum cellulose (Sph-C) and Sphagnum nanocellulose (Sph-NC) were characterized by Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), thermogravimetric analysis (TGA), dynamic light scattering (DLS), scanning electronic microscope (SEM) and transmission electronic microscopy (TEM). The mechanical properties of the Sph-NC/sodium alginate composite film were tested. FTIR showed that the non-cellulose impurities could be effectively removed from the Sphagnum by the alkaline treatment. XRD showed that the crystalline forms of cellulose in Sph-C and Sph-NC were both cellulose I. In the TGA, Sph-NC showed reduced thermostability whereas a higher residual mass. TEM observations revealed that the diameter of the prepared Sph-NC was approximately 21.5 nm, falling within the range of nanomaterial sizes. The tensile index and bursting index of the sodium alginate composite films increased by 21.2% and 15.7%, respectively, upon the addition of Sph-NC during the preparation process. It is concluded that Sphagnum can be used as a source of cellulose to prepare nanocellulose and the prepared Sph-NC plays a reinforcing effect on the sodium alginate composite films.