The invention discloses a preparing method of synthetic gas through reforming the by-product glycerin steam of biological diesel in the preparing technology of synthetic gas, which comprises the following steps: filling Ni/ZrO2 or Ni/CeO2 or Ni/CeO2-ZrO2 activator into reactor; setting the content of nickel at 10%-15%; adopting hydrogen gas and nitrogen gaseous mixture; proceeding reducing pre-treatment for the activator; leading glycerin water solution steam into the reactor with air speed at 5000-20000h-1 and ratio of water and carbon at 1:1-5:1; steam-reforming under normal pressure and with temperature at 300-600 deg. c; preparing the product. This invention possesses the advantages of simple craft course, low cost and wide prospect, which is fit for the requirement of green chemistry.