Abstract: The loop reactors are applied widely for its simple structure and many merits. To introduce the loop reactor for coal liquefaction，a large-scale loop reactor cold model experimental apparatus was built. Many tests for nitrogen-water system were made with this apparatus，and the effects of gas superficial velocity，pressure and superficial tension on gas holdup were investigated. The gas holdups of bubble state and loop state were compared and loop reactor’s operational flexibility was investigated. Important conclusions were reached.