2",3-Dihydroxy-4"-methoxychalcone was synthesized by the condensation reaction with 3-hydroxybenzaldehyde and paeonol occurred under the alkaline catalysis, this product molecular structure was determined by comprehensive analysis such as UV, IR, elemental analysis and NMR including of ¹HMNR, ¹³C NMR and 135^°dept, its yield was 82.2%, then and 7-methoxy-3"-hydroxyflavone was synthesized by base iodine catalyzed closing ring reactions to 2",3-dihydroxy-4"-methoxychalcone with DMSO as solvent at high temperature, the flavones" yield was 92.0%, this synthetic reaction could overcome the problem of protecting the hydroxyl group in the condensation reaction and realized the efficient synthesis of 7-methoxy-3"-hydroxyflavone. The changes of UV spectra of 2",3-dihydroxy-4"-methoxychalcone and 7-methoxy-3"-hydroxyflavone were investigated in acid, alkali and salt solutions of their different concentrations respectively, 2",3-dihydroxy-4"-methoxychalcone could react in HCl solution of 4 mol?L^-1 concentration, and also reacted in low-concentration NaOH solution, its variable substance could exist stably within 30 min, it easily reacted in KNO3 solution, the degree of reaction increased with the increase of the concentration of KNO3 solution and this variable substance could not exist stably for a long time. 7-methoxy-3"-hydroxyflavone could exist stably in HCl, NaOH and KNO3 solutions of their different concentrations for a long time.