Provided is a radioactive microsphere including glass having a structure represented by a formula Ca.sub.3Si.sub.2O.sub.7 and yttrium oxide contained in the glass. The radioactive microsphere has sphericity of from 0.71 to 1, and is radioactive after being activated by neutron irradiation. A method for preparing a radioactive microsphere and a radioactive filler composition is further provided. The present disclosure can be used to treat tumor by delivering radioactive microspheres to the target tissue, and then radioactive microspheres are activated by neutrons to generate radiation. The radioactivity of microspheres disappears over time, and the microspheres were dissolved and absorbed by the bone tissue in the end.