Sihetun is the most famous locality of the "Jehol Biota" with a large number of well-preserved fossils. To explore into the cause of bird mass mortality in the early Early Cretaceous in the Sihetun area, the authors have analyzed the multiphase correlation elements in the fossil samples from the bird-bearing beds, their top and floor plates, sedimentary strata of no-bird beds and volcanic rocks under the bird-bearing beds on the bases of the element geochemistry characteristics and geological data. The results show that the noxious gas containing SO2, H2S, PbBr2, PbBrCl, Pb(OH)Br, (PbO)2, PbBr2 and Hg is the major cause for bird mass mortality; the concentration of bird fossils results from volcanic eruptions from a volcanic crater near Sihetun.