The invention relates to a testing device and a testing method for the thermal conductivity coefficient of a material. The testing device comprises a hot plate heating system, a cold plate cooling system, an electric lifting system, a heat flow metering system, an electrical control and measurement system and an industrial personal computer software control system. The hot plate heating system is horizontally arranged, a core plate, a preventive plate and a back plate of the hot plate heating system respectively comprise a heating resistance sheet and a metal panel, the external dimension of the preventive plate is 550mm*550mm, the electric lifting system comprises a fulcrum bar, a lifting bracket, a lead screw and a driving motor, the lifting fulcrum bar drives a hot plate to lift, and the lifting fulcrum bar and the hot plate can be detached. The testing device for the thermal conductivity coefficient of the material is suitable for testing test pieces with thicknesses less than 100mm, and a building slab type heat-insulating material with the thermal resistance more than 0.1m<2>.K/W, and is high in temperature control precision, short in detection period and accurate in test result, can effectively avoid the influence on the edge hot loss and the drift of a heat-flow meter, and flexible materials are easily tested.