Overhigh temperature can induce inflammation and heat radiation damage to normal tissues around the tumor. Therefore, the development of a magnetic material that can achieve high mortality of tumor cells at relatively low temperatures (e.g. 43 ℃) is critical for the clinical application of magnetocaloric therapy. This study focuses on the goal of low-temperature, safe, and effective magnetic thermotherapy. The liquid-solid phase transition material polylactic acid-glycolic acid (PLGA) approved by the FDA, is selected as raw material and superparamagnetic iron oxide nanoparticles prepared by a one-step mild reduction method are loaded to achieve magnetic resonance imaging and magnetic heating. Meanwhile, a small molecule inhibitor of heat shock protein HSP90, i.e, epigallocatechin gallate (EGCG), encapsulated in PLGA, could inhibit the body’s thermal protection function and achieve the purpose of killing tumor cells at lower temperatures. In vitro results indicated that the prepared superparamagnetic iron oxide nanoparticles not only have good T₂-weighted imaging performance, but also show excellent magnetothermal transformation performance. More details, when the obtained PLGA/Fe₃O₄/EGCG biocomposite was controlled to heated up to 43 ℃ for 40 min under an alternating magnetic field, around 70% of tumor cell could be killed, exhibiting promising potential for low-temperature magnetothermal treatment of tumor. Such novel injectable magnetic thermal phase transition material may provide new idea and material support for the treatment of osteosarcoma.