In order to improve mechanical properties of polyether polyurethane elastomer (PU), three kinds of alkyl-terminated hyperbranched polyesters (HBP-C6, HBP-C12, HBP-C18) were introduced by interpenetrating polymer network (IPN) modification technique. The results show that mechanical properties of IPN are improved when the content of HBP is low. The longer the end-chain alkanes of hyperbranched polyesters is, the smaller the phase domain size becomes. When the content of HBP-C18 is high, both the tensile strength and the elongation at break of IPN increase first and then decrease. When the content of HBP-C18 is 30%, the dual continuous interpenetrating networks are observed in IPN. The tensile strength is 1.76 times than that of PU, and the elongation at break is as high as 2055%, 7.56 times higher than before. The effect of synthetic method of HBP-C18 such as pseudo one-step method and one-step method on mechanical properties of IPN was also studied. The results show that the former performs better mechanical properties.