In this work, the thermal cracking of n-decane was carried out under supercritical pressure. To comprehensively investigate the thermal cracking, a developed online-sampling experimental method was constructed through the directly cooling of cracked fuel at high temperature. The results show, that the novel method can effectively terminate the subsequent cracking which occurred in the heat exchanger. Therefore, compared with the traditional offline-sampling method, lower gas product yields and conversions were obtained with this novel method. Besides, products composition and distribution were detected and analyzed in detail. In gas products, more alkanes and hydrogen were obtained while olefins were relatively less according to our novel method. For liquid products, similar composition and distribution can be obtained in both methods except that aromatic products decrease significantly by adopting the novel method. In general, this novel method can provide precise product yield and composition with little influence on the thermal cracking process.