The purpose of this study was to clarify the population dynamics of Aphis rumicis and Menochilus sexmaculata on Rumex dapibus herba, and the effects of Aphis rumicis on the growth, development and fecundity of the dominant natural enemy, Menochilus sexmaculata. In this study, the Rumex dapibus herba was used as the host plant of A. rumicis, and the population dynamics of aphid and ladybeetle were investigated every 6 days by using the five-point sampling method. Ladybeetle was fed with aphid, and the developmental duration, survival rate and oviposition of adults were recorded. The results showed that: (1) the maximum population density of A. rumicis appeared in late July, which was 35.29 individuals/plant, and the maximum population density of M. sexmaculata in the field survey of leaf-eating grass planting base during the same period was 6 days later than that of A. rumicis, and the maximum population density was 1.60 individuals/plant. (2) During the whole development of the laboratory population, the survival rates of the egg stage, the 1-4 instar larval stage, and the pupal stage (pre-pupal stage) were 93.3%, 87.5%, 96.0%, 97.9%, 100%, and 100%, respectively, and the overall survival rate from eggs to the completion of one generation was 66.7%. The development period from egg to pupal was (2.49± 0.07) d, (2.75±0.07) d, (2.38±0.01) d, (2.40±0.07) d, (2.05±0.03) d, (5.62±0.10) d, and the pre-oviposition period was (2.52±0.10) d. (3) The net reproduction rate R₀=108.27>1, the intrinsic growth rate r=0.18>0, and the weekly growth rate λ=1.20>1, showing an increasing trend. Average generational duration = 25.95 d. The above indicators showed that the population of ladybeetle was increasing, which proved that Rumex dapibus herba was also a good food source for A. rumicis, and A. rumicis could be used to rear M. sexmaculata.