A significant gap remains in our understanding of the systemic responses of ferns to multiple stress factors and the application of biostimulants. Effects of single drought stress (DS), salinity stress (CaCl2), combined stress (CaCl2+DS) and depolymerized sodium alginate (DA) on morphology and physiology characteristics of the fern Athyrium nipponicum cv. Metallicum in a pot experimental were investigated. Plants grown in a plastic tunnel were treated with CaCl2 at a target concentration of 100 mmol dm-3 and DA at 100 mg dm-3 under normal irrigation and drought stress conditions. Combined stress (CaCl2+DS) most severely reduced plant height, plant width, leaf number, and leaf length. Plant leaf and rhizomes weights were significantly reduced under water deficit stress than excess soluble salts in the substrate. Plants exposed to CaCl2 had elevated leaf carotenoids (by 40.9%), total polyphenols (by 22.1%) and total flavonoids (by 15.0%) content relative to control plants. The application of DA had no apparent effect on the aboveground part of the plants, while it increased the fresh and dry weights of the rhizomes by 48.1% and 51.0%, respectively. Moreover, DA application increased chlorophyll a, chlorophyll b, and total chlorophyll contents of leaves by 53.0%, 56.3%, and 53.1%, respectively, compared to the control. Application of DA under single (CaCl2+DA and DS+DA) and combined (CaCl2+DS+DA) stress conditions significantly reduced the fresh and dry weights loss of rhizomes. Comparing the content of secondary metabolites in fern, rhizomes had 4.8 times more total polyphenols and 3.6 times more total flavonoids than leaves. The results suggest the potential application of DA in mitigating the adverse effects of salinity and water stress during fern cultivation.