The invention discloses a natural circulation loop system of a lead-bismuth alloy. The system comprises a storage tank, a first subchannel, a visual pipeline, a rising pipeline, a condenser, a water pump, a top pipeline, a descending pipeline, a second subchannel and an electric-heating section. The system has the characteristics of being simple in structure and simple and convenient to operate. Based on the secondary heat exchange of the condenser and the electric-heating section, the heat exchange effect of the system is good; pressure gauges and temperature instruments are arranged on all the pipelines of the system while the pipelines are controlled by a valve, so that the safety coefficient of the system is high; the cooling in nuclear industry is carried out by a natural circulation loop, so that the heat energy generated by the nuclear industry is utilized effectively, i.e., the waste of energy sources is avoided. The system not only can be applied to the nuclear industry but also can be applied to scientific research. As a result, the system disclosed by the invention can serve as a practical tool and a scientific-research tool simultaneously.