A kinetic model was proposed to describe hyperbranched polymers (HBPs) formed by the polymerization of monomers A(2)and B(3)with monofunctional compound (BR) added gradually in a semibatch reactor. The dependences of the degree of polymerization (DP) and the degree of branching on the reaction time and the monomer compositions were calculated. The DP of the HBPs could be increased by the slow addition of BR in the reactor. If the monomers B(3)and A(2)were mixed at a molar ratio of B-3:A(2)= 1:3, and the BR was fed slowly into the reactor, the HBPs with weight average DP were approximately 3200 and 16 700 at conversion of 0.95 and 0.99, respectively, which were higher than those prepared by the batch system. This theory was further verified by an experimental demonstration. Hyperbranched polyurethane acrylate can be synthesized with a higher DP in a semibatch system than in a batch reactor.