The invention discloses a preparation method of nano cuprous oxide. In the method, copper chloride and copper nitrate are taken as copper sources, hydrazine hydrate, glucose and an ascorbic acid and the like are taken as reducing agents, and cuprous oxide materials in the shape of an octahedron, a hollow sphere, a cube and a hollow cube and the like are successfully obtained by using a liquid-phase reduction method and a hydrothermal method. Required raw materials for preparation are rich, the cost is low, and no waste is produced. The preparation process is simple, and the repeatability is good. Cuprous oxide nano-materials have wide applications in the aspects of photocatalysis, gas-sensing materials, antibacterial materials, and the like.