The invention belongs to the technical field of nuclear reactor engineering, and in particular relates to a thermally-operated conversion system which adopts lead or lead-bismuth alloy intermediate loops. The system comprises three loops; a first loop comprises a cold pool, a main pump, a reactor core, a thermal pool and an intermediate heat exchanger arranged in a main container and connected in sequence; a second loop comprises a second loop circulating pump, the intermediate heat exchanger and a steam generator connected in sequence; a third loop comprises a water supplying pump, the steam generator and a steam turbine connected in sequence; a working medium of the first loop is liquid metallic sodium, a working medium of the second loop is liquid metallic lead or a liquid lead-bismuth alloy, and the third loop is a conventional water-steam loop. By adopting the system disclosed by the invention, heat generated by the reactor core can be safely and efficiently brought out of a reactor, so that high-temperature steam is generated in the steam generator and used for pushing the steam turbine to apply work, and heat energy generated by the reactor is converted into mechanical energy.