Aims <p>As the population continues to grow, increasing demand for livestock products is driving the development of livestock systems worldwide. Intensive cultivation by diversifying sowing dates has gradually become an important option to meet the increasing demand for food. The purpose of this study is to investigate how the productivity of the mixed-cropping system changes under the adjustment of the sowing season.</p> Methods <p>We investigated the effects of spring or summer sowing season on forage productivity and interspecific relationship in an oat and common vetch mixed-cropping system, based on a two-year field experiment conducted in northwestern China.</p> Results <p>Our results show that no significant difference was observed in the forage biomass and land equivalent ratio (LER) of mixed-cropping system among different sowing dates. However, altering sowing season could pose obvious impact on the forage quality. Over the experimental period, the average crude protein (CP) content and relative feed value (RFV) were 9.0% and 12.9% lower in spring sowing compared to that in summer sowing. Furthermore, compared with the monoculture system, the mixed-cropping system with the combination of spring and summer sowing seasons increased average annual system forage productivity and economic benefit by 37.0% and 49.9%, respectively, during the whole experimental period.</p> Conclusions <p>Mixed cropping with the combination of spring and summer sowing seasons is recommended as a sustainable agricultural system in the northwestern of China, in order to alleviate the seasonal impact on the forage productivity and interspecific competition, ensuring annual high-quality forage productivity stability.</p>