Abstract: Effectively inhibiting intracellular nuclease activity plays a key role in the enhancement of integrating exogenous DNA and stable expression.By research on intracellular nuclease activity in three strains, that is, A9 strain, A9c strain (mutant anti2L canavanine sulphate) and A9L strain (elongated strain), it was found that intracellular nuclease activity could be inhibited by adding ethylene diamine tetraacetic acid in the culture media or being treated for variant timing at variant temperature, but proper consistency of ethylene diamine tetraacetic acid and appropriate temperature and time were key factors to keep the three strains being normal physiological status.The intracellular nuclease activity in A9 strain and A9c strain was obviously inhibited with 2mmol/L EDTA or with the treatment for the filament at 45 ℃for 15 min.With this treatment A9 and A9c filament appeared to be in normal physiological state.As for A9L strain, 2mmol/L EDTA or dealing with filament at 50 ℃for 5 min obviously inhibited the intracellular nuclease activity, and the treatment could let the filament still in its normal physiological state.By the fundamental and comparative study of the intra cellular nuclease activity in different strains, we could come to the conclusion that S.platensis A9 strain, A9c strain and A9L strain may be served as receptors for genetic transformation, some proper methods could be used to inhibit the intra2 cellular nuclease activity, and the technique might help the integration and expression of the exogenous gene in Spirulina.