As dispensable material to prepare polylactide, the purity and cost of lactide effect the quality and price of polylactide. The oligomer dehydrated from oligomer in last reaction can be used as reaction stuff like fresh lactic acid, but the suitable react condition is not clear. This paper researches the difference between react system of lactic acid and oligomer, including the level of average molecular weight, relative content of composition and the influence of added catalyst. The maximal yield of lactide could reach 28%.