Disclosed is a liquid-phase oxidative decomposition method for radioactively contaminated carbonaceous material, providing a method of oxidizing carbon into a gas in liquid phase to treat radioactively contaminated carbonaceous material. The method comprises the following steps: ball milling a mixture of a molybdenum-containing substance and a carbonaceous material, thermally treating the ball milled mixture, and performing liquid-phase oxidation of the thermally treated mixture. The thermal treatment causes carbon to enter space between molybdenum atoms so as to reduce the particle size of carbon and improve the chemical reactivity of carbon, and an oxidant is then used to oxidize the carbon in the space between molybdenum atoms into a gas in liquid phase, while the molybdenum-containing moiety is converted into a water-soluble substance. The method of has technical effects of mild reaction conditions, low energy consumption, high operation safety, and facilitates the recovery of elements attached to carbonaceous material.