The stems of Allium victorialis L. var. platyphyllum were extracted and the major essential oil composition and immunotoxicity effects were studied. The analyses were conducted by gas chromatography and mass spectroscopy (GC-MS), which revealed the essential oils of A. victorialis L. var. platyphyllum stems. The A. victorialis L. var. platyphyllum essential oil yield was 1.45%, and GC/MS analysis revealed that its major constituents were allyl methyl disulfide (24.36%), dimethyl trisulfide (11.78%), allyl cis-1-propenyl disulfide (9.17%), allyl methyl trisulfide (4.13%), and dipropyl trisulfide (7.22%). The essential oil had a significant toxic effect against early fourth-stage larvae of Aedes aegypti L. with an LC(50) value of 24.12 ppm and an LC(90) value of 34.67 ppm. Also, allyl methyl disulfide (≥95.0%), dimethyl trisulfide (≥95.0%), allyl cis-1-propenyl disulfide (≥95.0%), allyl methyl trisulfide (≥95.0%), and dipropyl trisulfide (≥95.0%) were tested against the F(21) laboratory strain of A. aegypti. Allyl cis-1-propenyl disulfide (≥95.0%) has good activity with an LC(50) value of 15.35 ppm. Also, the above data indicate that other major compounds may play a more important role in the toxicity of essential oils.