The two-component polyurethane coating film is formed by the chemical reaction of hydroxyl resin and polyisocyanate,which is widely used in the coating of automobile,engineering machinery,buses,rail trains,etc.In order to obtain good hardness,water resistance,solvent resistance and scratch resistance,the gelation reaction of the system is necessary.Based on the Carothers gel-point theory,the chemical reaction of two-component polyurethane system was deeply analyzed.The influence of the functional degree of hydroxyl resin composition on the critical reaction degree of the system was emphatically analyzed and the influence of the relative dosage of the two components on the molecular weight and crosslinking density of the system was also analyzed.The results are of great importance for the understanding of the film forming mechanism of the system and optimization of the product composition.