The invention discloses streptococcus suis truncated protein Sao and application thereof. Amino acid sequence of the streptococcus suis truncated protein Sao is as shown in the SEQ ID NO.2. The protein stability is better than stability of full-length protein, and the protein of the invention is not easy to degrade. After the Sao protein is used for preparation of a vaccine, the vaccine can effectively protect piglets. By the use of large dose of virulent strain SC19 for counteracting toxic substances, the protecting force still reaches 60%. The Sao protein is safe and nontoxic. After immunization of piglets and pregnant sows, body temperature, breathe, appetite and metal state are all normal during the whole observation period, farrowing of pregnant sows is normal, and there is no abortion, fetal death and the like. The Sao protein can be combined with other proteins to prepare multi-vaccine and is suitable for mass production.