Through thermodynamic calculation and experiment analysis, the cause of impurity forming in preparing La1-xSrxMnO3 by self-propagating high-temperature synthesis (SHS) was studied and the suggestion was given in this paper to avoid or diminish it. The results show that La2SrO4 impurity was produced inevitably in the products synthesized by SHS because the Gibbs free energy of La2SrO4 forming reaction is equivalent to that of SrMnO3 forming reaction when MnO2 was adopted as dilution. When La1-xSrxMnO3 was used as dilution, La2SrO4 forming could be avoided. The over-dilution of La1-xSrxMnO3 resulted in Mn3O4 impurity forming, which could be avoided by adjusting the proportion of dilution and controlling the reaction speed and system temperature of SHS process.