The reaction course and delignification kinetics of wheat straw pretreated with propionic acid was studied. The results showed that two distinct delignification phases occurred during this pulping process of wheat straw, which were the initial delignification phase from the beginning to 60min and the residual delignification phase from 60min to 180min. The rate constant and the order of reaction were determined through the establishment of kinetic equation in delignification reaction. Under the temperature of 333K,343K,353K,363K, the rate constants were 0.0023, 0.0028, 0.0037 and 0.005. And the reaction order was the first grade. In the range of temperature and time under which the experiment was carried out, the average active energy was 26.05kJ/mol, the frequency constant was 26.797L/(mol·min).