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The aphid parasitoid <i>Aphelinus asychis</i> Walker is an important biological control agent against many aphid species. In this study, we examined whether the rearing host aphid species (the pea aphid, <i>Acyrthosiphon pisum</i> and the grain aphid, <i>Sitobion avenae</i>) affect the performance of <i>A. asychis</i>. We found that <i>A. pisum</i>-reared <i>A. asychis</i> showed a significantly larger body size (body length and hind tibia length) and shorter developmental time than <i>S. avenae</i>-reared <i>A. asychis</i>. There was no difference in the sex ratio between them. The longevity of <i>A. pisum</i>-reared <i>A. asychis</i> was also significantly longer than that of <i>S. aveane</i>-reared <i>A. asychis</i>. Furthermore, <i>A. pisum</i>-reared <i>A. asychis</i> presented stronger parasitic capacity and starvation resistance than <i>S. aveane</i>-reared <i>A. asychi</i>. In addition, host aphid alteration experiments showed that <i>A. asychis</i> only takes two generations to adapt to its new host. Taken together, these results revealed that <i>A. pisum</i> is a better alternative host aphid for mass-rearing and releasing of <i>A. asychis</i>. The body size plasticity of <i>A. asychis</i> is also discussed.