The divertor target is one of the main components enduring the high heat load in fusion reactor. The life time and integrity of the divertor are severely limited by the failure of the target heat sink especially in operation of high temperature and long operating period of China Fusion Engineering Test Reactor (CFETR). Therefore, the study of the failure mechanism of heat sink under long pulse and high-power H-mode operation is one of the bases to ensure the reliability of CFETR. In this paper, the finite element method is used to analyze the structural response of the heat sink to the thermal load of the W/RAFM(Reduced Activation Ferritic-Martensitic) divertor target under CFETR design operating conditions. Then the numerical creep-fatigue damage accumulation model is built to explore the fatigue failure mechanism of divertor target heat sink in high power / long pulse operation during which the stress relaxation is taken into account. The preliminary creep-fatigue assessment of CLAM steel as divertor target heat sink in CFETR design operation phase is presented and provides important theoretical basis for life prediction of heat sink for future fusion reactor.