The present invention relates to a biodegradable and thermosensitive poly(organophosphazene) with a functional group, a preparation method thereof, and a use thereof for delivery of bioactive substances. According to the present invention, poly(organophosphazene) is a phosphagen-based polymer showing biodegradability, thermosensitivity, and sol-gel phase transition depending on temperature change, whereby when administered into a living body with bioactive substances such as drugs, the poly(organophosphazene) forms a gel-phase at body temperature to be capable of controlled release of the bioactive substances. Further, the poly(organophosphazene) has functional groups to chemically bind with bioactive substances through an ionic bond, covalent bond, or coordinate covalent bond to be capable of a sustained release of the bioactive substances due to its good binding property. Therefore, the poly(organophosphazene) is useful as a delivery material for bioactive substances.