To assess the physico-chemical properties and the anaerobic biodegradability of S. alterniflora harvested at different growth stages, a wet mesophilic anaerobic experiment was performed. With the extension of growth period, the content of easily biodegradable organic matter decreased while lignocellulose content and C/N ratio increased, moreover, lignocellulosic structure became mature and the crystalinity of cellulose decreased which made the biodegradability of S. alterniflora decreased with the growth stage. However, the metal cations content decreased with the growth stage, it meant that the possibility of inhibitory to anaerobic digestion was decreased. The biotransformation rate was rather low when S. alterniflora was directly digested. The highest value of 42.55% was obtained when S. alterniflora was collected at May and the lowest value of 18.76% by August. Taking the biomass and the biogas stability into consideration, August is the optimum stage for harvest.