In order to investigate the basic mechanical performances of the composite girder under the fire loads, three typical experimental models including simple-support T-section girder, simple-support box-section girder, and continuous box-section girder, were built. After establishing a detailed experimental plan, a fire experiment loaded on three sides was conducted in a laboratory fire furnace. The comparative study on the material characteristics of steel between the models before and after the fire loads showed that the yield and limit strengths decreased by 10%~20 % after fire loads with the 700~900 ℃.The elastic modulus of the steel girder only slightly changed with the increase of temperature. The influence on the concrete material during the fire could be almost ignored; however, 5% decrease of the concrete material strength could be found after cooling off. The results provided good experimental and theoretical foundation for determining the basic material characteristics in the anti-fire design of the composite girders.