A new organotin initiator with amino-terminated hyperbranched polysiloxane (HSiSn) for the ring-opening polymerization of cyclic butylene terephthalate (CBT) oligomer was synthesized. Compared with a traditional initiator, butyltin chloride dihydroxide (BCD), HSiSn has a moderate initiate speed, lower toxicity, and good reactivity. Poly(butylene terephthalate) (PBT) is the ring-opening polymer of CBT, of which the crystalline and molecular weight are almost independent of the initiator used, but the PBT initiated by HSiSn has better thermal stability than that by BCD. On this basis, a series of new toughened bismaleimide (BD) resins (HSiSn/CBT-BD) were prepared through the in situ formation of PBT during the prepolymerization of BD resin with HSiSn. Compared with BCD/CBT-BD and BD resins, HSiSn/CBT-BD resins with small loadings of CBT (<= 5 wt %) have remarkably improved integrated performances, including higher impact strengths, improved flexural, and tensile properties, better dielectric properties, and excellent heat resistance. These attractive performances are attributed to the unique cross-linked structure induced by HSiSn.