Buffer gas-assisted high-irradiance laser ionization time-of-flight mass spectrometry (BGA-LI-TOFMS) was used to determine the elemental distribution of Lycoptera fossils from the Dawangzhangzi Bed of the Yixian Formation in Liaoning Province, China. The Sr, Ba, and Pb images of the Lycoptera fossil highlight the fine structure of the skeleton, and the distribution of Fe and Si provides data regarding the preservation pattern and nature of pyritization in the Jehol Biota. The enrichment of toxic substances such As and H2S may be one of the reasons for the cluster mortality of Lycoptera. Meanwhile, the elemental analysis of the rocks in the fossil preservation shows that the paleoclimate indices MgO/CaO and Sr/Cu indicate dry and hot climatic conditions at that time. Meanwhile, the paleosalinity indicators (m value and Sr/Ba ratio) indicate a semi-saline to slightly saline water body. The paleoredox environment indicators (Th/U and V/(V+Ni) ratio) suggest an anaerobic environment with weak stratification and H2S in the bottom water. Thus, the elemental imaging and analysis of the Lycoptera fossil suggest that the organisms of Jehol Biota were preserved in the anoxic water environment and the toxic effect of hydrogen sulfide. Overall, BGA-LI-TOFMS is an adaptable technique for acquiring comprehensive geochemical information on fossils.