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Simple Summary: Sophoridine, a natural quinolizidine alkaloid with biological activities, was isolated and identified from traditional medicinal herbs. Although mosquitoes can be effectively controlled by chemical insecticides, their intense use has significantly increased the development and spread of resistant mosquitoes. Chemical insecticides have negative effects, such as potential health risks, water contamination, environmental pollution, and toxicity to nontarget organisms, including people. In this study, two series of sophoridine derivatives were generated using sophoridine as the lead compound. All derivatives were characterized and screened for their insecticidal activity against larva and adult mosquitoes. Their effects on the growth cycle of larva and their action on AChE from Aedes albopictus were also explored. Finally, morphological studies of the derivative-treated dead larvae were conducted. Two series of novel sophoridine derivatives were designed, synthesized, and evaluated for their anti-mosquito activity. SOP-2g, SOP-2q, and SOP-2r exhibited potential larvicidal activity against Aedes albopictus larva with LC50 values of 330.98, 430.53, and 411.09 ppm, respectively. Analysis of structure–activity relationships indicated that the oxime ester group was beneficial for improving the larvicidal biological activity, whereas the long-chain aliphatic group and fused-ring group were introduced. Furthermore, the larvicidal mechanism was also investigated based on the inhibition assay of acetylcholinesterase (AChE) and the morphological observation of dead larva treated with derivatives. Results indicated that the AChE inhibitory activity of the preferred three derivatives were 63.16%, 46.67%, and 35.11%, respectively, at 250 ppm concentration. Additionally, morphological evidence demonstrated that SOP-2q and SOP-2r induced changes in the larva's intestinal cavity, caudal gill, and tail, thereby displaying larvicidal action against Ae. albopictus together with AChE inhibition. Therefore, this study implied that sophoridine and its novel derivatives could be used to control the population of mosquito larva, which may also be effective alkaloids to reduce the mosquito population density. [ABSTRACT FROM AUTHOR]