Abstract: With Bifidobacterium longum as experimental strain,in polymer γ- PGA as main protective agent,freezedrying experiment was carried out.The optimal formulation of protective agent was studied by survival rate,the ability of producing acid,the morphology of the protective agent and the cell morphology that was repeated freezing and thawing after 3 times.The results showed that using 10% dried skim milk as a protectant cell survival rate was 47.3%,with 2.5%γ- PGA as a protectant cell survival rate was 54.8%.The survival rate of compound that1.5%γ- PGA mixed with 10% Skim Milk Power was up to 79.8%,which was about 25.0% higher than using the single dried skim milk protectant at 10%.Electron microscope results showed that,with composite protective agent bacteria surface form a dense structure.After repeat freezing and thawing 3 times,there was no serious damage on bacteria.With strains that freeze- dried bacteria powder watered 2 h for fermentation,the fermentation acidity was65.5 °T after 18 h.The survival rate stored 90 d at room temperature could reach 48.7%.