Provided is a composite microparticle, composing a hydroxyapatite based core, and a cover layer covering the hydroxyapatite based core by which an active component can be protected in acidic environment, wherein the cover layer is alginate. The invention also provides a method for preparing the composite microparticle.一種複合微粒,係包括:氫氧基磷灰石核芯;以及包覆該氫氧基磷灰石核芯之覆蓋層,其中,該覆蓋層係海藻酸鹽,俾透過該覆蓋層達到控制釋放活性成分的效果。本發明復提供該複合微粒之製法。