The invention discloses a method for testing thermal property of semicokes generated by cracking of non-coked block coal. The method comprises the steps of: preparing the semicokes from the non-coked block coal; gasifying the semicokes by mixed gas of CO2 and reductive gas, detecting the ingredients of tail gas and drawing a curve chart of reaction time-CO concentration; performing a drum test on the gasified semicokes, and sieving the coke samples after the drum test by sieves with different mesh diameters; drawing a cumulative distribution diagram of the mass percentage of each granularity grade; weighting and summing the mass percentage and the weight of each granularity grade to obtain after-reaction strength. The device for testing the thermal property mainly comprises an electric furnace, a temperature control cabinet, a reaction tank, a flue gas analyzer, a computer, a gas storage tank, and a drying tower and a flow meter which are communicated with the gas storage tank. The problems that the after-reaction strength is low, the evaluation of the thermal property of the semicokes with wide granularity distribution is not correct, the reaction process is not detailed and the like in the traditional method are solved, and the evaluation of the thermal property of the semicockes which are prepared from the non-cake block coal and are gasified under the reductive atmosphere is realized.