Abstract: In order to compensate the reactivity depleted due to burnup and reduce the dependence on the beam intensity of accelerator, a design scheme of ADS with fast thermal unidirectional coupled subcritical core, liquid lead-bismuth alloy single circuit cooling and graphite moderating was given. The use of thorium fuel and the conversion of thorium to uranium were taken into account. The results indicate that the power flattening, neutron unidirectional coupling and extending the refueling cycle are achieved and the adverse effects of the cavity are eliminated. The temperature reactivity coefficient of the final design reactor is negative, and the safety is high. In addition the core has a high energy amplification capability, and the change of k^eff is not more than 1.05% in the lifetime. The maximum beam intensity required for the accelerator is 4.21 mA. Besides, the transmutation support ratio of the core is 15 PWRs of million kilowatt, and the transmutation ability is strong.