N, N-diethyl-m-toluamide (DEET) is a widely used active ingredient in insect repellents, particularly in areas where mosquitoes are prevalent. Although the photolysis of DEET is inevitably to occur when exposed to sunlight, the potential risks associated with its transformation products have not been extensively studied. In this study, multiple in-vitro bioassays were conducted to examine the microtoxicity, genotoxicity and estrogenic effects of these products. Subsequently, ultraperformance liquid chromatography coupled with triple time-of-flight mass spectrometry (UPLC-TripleTOF) was adopted to identify the molecular structures of the main photolysis products. Ultimately, the correlation between the products and their toxicities was analyzed to spot the compounds potentially responsible for the elevated toxicity. The results showed that although DEET was only moderately degraded under sunlight, with a 20% reduction after photolysis, the microtoxicity was increased by 83.3%, indicating the formation of toxic products. The characterization results revealed that eight main photolysis products were generated via de-alkylation, oxidation, and electrophilic substitution. Among these, six products containing the N-ethyl-m-toluamide functional group were found to be microtoxic. Fortunately, no genotoxic or estrogenic products were produced during DEET photolysis with simulated sunlight. These noteworthy findings emphasized the potential risks associated with the use of DEET-containing topical insect repellents when exposed to sunlight.