Abstract: Thymosin α1 (Tα1) is a 28 amino acid peptide, which as immune system enhancer.It had been expressed and purified in a kind of blue green algae, Spirulina platensis.The expression would produce a valuable and cheaper bio—product, because it omitted an expensive purifying process and can be used as an oral medicine or feed additive directly.In this study, A cluster expression system was designed and a gene integration platform system for Spirulina platensis was constructed.ELISA and HPLC analysis was used to compare the difference of expression amount among the 1 to 4 Tα1) gene cluster in transgenic Spirulina platensis.The results showed that the cluster expression was effective and the maximum amount of Tα1 reached 7% of total protein.The immunity enhancement by the transgenic Spirulina platensis was demonstrated by feeding Anguilla japnica as feed additive.Tα1 contents in the tested tissues and organs of the black fry increased after fed with transgenic algae.The transgenic immunity-stimulated Spirulina platensis was the feasible additive of formulated feed for farming fish and its additive dosages was about 0.5—1.0 g·kg-1.