The invention discloses a rice field milk vetch sowing-free circulation seed reserving method. The method comprises the following steps that after milk vetch seeds are mechanically harvested in early and mid May, smashed milk vetch straws are evenly spread in a rice field, irrigation is conducted, a drainage outlet is blocked (so as to prevent milk vetch seeds reserved in the rice field from flowing away along with water), biodegradation is conducted for five to ten days, dry farming is conducted to achieve single-season rice transplantation or single-season rice no-tillage direct sowing, the single-season rice is irrigated by shallow water at the early growth stage, water falling and field drying are conducted at the rice milk-ripe stage, the single-season rice is harvested from the mid and late September to early October, high stubbles of straws are reserved to be returned to the field, or removed from the seed reserving field within one to three days, ditching or ditch cleaning is conducted, and no water is reserved in the field in sunny days or accumulated in rainy days; routine management of milk vetch planting is conducted, and circulation operations of farming work such as milk vetch seed mechanical collection and single-season rice sowing are conducted in the next early and mid May. The rice field milk vetch sowing-free circulation seed reserving method solves the problems that at present, field abandonment is general in winter in the single-season rice field, and rice field green manure milk vetch seeds are scarce, and relieves milk vetch seed shortage, increases milk vetch seed reserve, and achieves the effect of returning milk vetch straws for fertilizing rice field soil.