Abstract: Pyrethrins are highly effective, low-toxicity, and environmentally friendly natural insecticides with good bioactivity against aphids. However, their susceptibility to photodegradation reduces insecticidal activity, thereby limiting field application. To enhance their stability and efficacy, the star polycation (SPc) was employed as a nanocarrier to construct a pyrethrin/SPc nano-pesticide (PY@SPc). The characteristics of PY@SPc were systematically evaluated, and its control efficacy against aphids was tested under both laboratory and field conditions. The results showed that the pesticide loading content of SPc for pyrethrin was 46.86% (freeze-drying method) and 44.22% (high-performance liquid chromatography, HPLC). The encapsulation efficiency was 88.44% (freeze-drying method) and 90.25% (HPLC). Compared with the pyrethrin solution without SPc, PY@SPc exhibited a decrease in particle size from 186.32 nm to 72.00 nm, an increase in zeta potential from 7.77 mV to 20.20 mV, and a reduction in contact angle from 79.7° to 13.5°. UV stability tests confirmed that the photodegradation rate of PY@SPc was significantly slower than that of PY. Laboratory bioassays indicated that the toxicity of PY@SPc against wheat aphid (Schizaphis graminum) was higher than that of PY, but the difference was not statistically significant. Field trials further demonstrated that the control efficacy of PY@SPc against both wheat aphids (Sitobion avenae and Rhopalosiphum padi) and peanut aphids (Aphis craccivora) was significantly superior to that of PY and the positive control (10% imidacloprid WP), demonstrating remarkable quick-acting and sustained activity. In summary, PY@SPc showed significant improvements in dispersibility, stability, wettability, and resistance to UV degradation. These enhanced properties not only improved both initial efficacy and duration of control against aphids, but also provided an important basis for the optimization of pyrethrin formulations and their field application.