Using full-potential linear-muffin-tin-orbital method, we have performed molecular-dynamics simulations for the ethylene adsorption on the Si5-7-cluster surfaces. The calculations show that the most favored adsorption site is the short bridge site for Si-5 cluster, with the adsorption energy 1.78 eV. The adsorption structure of ethylene molecule is similar to that of the dimer-maintained structure for C2H4+Si(100)-(2x1). It indicates that ethylene is di-sigma bonded to the Si-5 cluster. At the same time, the Si-5 cluster reconstructs after ethylene adsorption. The three-center bond among side atoms breaks, and new bonds form. For Si-6 cluster, the most favored adsorption site is the atop site on the side atom. After adsorption, Si-6 cluster reconstructs from tetragonal bipyramid to edge-capped trigonal bipyramid. The short bridge is the only available site to adsorb ethylene for Si-7 cluster.