The invention provides quaternary phosphonium salt with a structural formula shown by formula I. The invention also provides application of the quaternary phosphonium salt as a reaction catalyst for preparing an amide/imide type polycarboxylic acid macromonomer. The quaternary phosphonium salt provided by the invention has the advantages of novel structure, large conjugate area, strong alkalinity (pKa=40-45) and high catalytic activity and applicability; the preparation method is simple and feasible, and a water carrying agent and a polymerization inhibitor are not needed in the reaction process; and moreover, the cost and energy consumption are low, and the method is environment-friendly and clean and has a prospect in industrial application.