Embolization is often used to block blood supply for controlling the growth of fibroids and malignant tumors, but limited by embolic agents lacking spontaneous targeting and post-treatment removal. So we first adopted nonionic poly(acrylamide-co-acrylonitrile) with an upper critical solution temperature (UCST) to build up self-localizing microcages by inverse emulsification. The results showed that these UCST-type microcages behaved with the appropriate phase-transition threshold value around 40 °C, and spontaneously underwent an expansion-fusion-fission cycle under the stimulus of mild temperature hyperthermia. Given the simultaneous local release of cargoes, this simple but smart microcage is expected to act as a multifunctional embolic agent for tumorous starving therapy, tumor chemotherapy, and imaging.