In this paper SrBi2Nb2O9 was prepared by molten salt synthesis method and KCl chosen as molten medium. In order to disclose possible formation mechanism of SrBi2Nb2O9, a simple route was established for comparison by dividing the starting precursors into four groups. With the help of XRD, possible synthesis route was analyzed. The results showed that pure SrBi2Nb2O9 powders could be obtained at temperature higher than 700oC by molten salt synthesis. Intense synthesis reaction happened in the temperature range from 500 to 700oC. The reaction mechanisms of SrBi2Nb2O9 included four steps, namely the formation of 1) SrBi2O4 at close to 600oC,2) BiNbO4 from the reaction of at 600-700 oC, 3) SrBi2Nb2O9 from the reaction of SrBi2O4 and Nb2O5 and 4)SrBi2Nb2O9 from the reaction of BiNbO4 and SrCO3. The molten salt could improve distribution and reactivity of the precursors and significant formation of SrBi2Nb2O9.