The computational model of numerical analysis of the planar pre-stressed steel trusses subjected to fire load is established according with main consideration to the response mechanism of the structure in fire. For the convenience of application, the EC3 constitutive model of the steel materials in fire is simplified with series expansion, and the suggested formula for each stage are unified. The fire behavior of the planar pre-stressed tubular steel trusses in which two cables are set is analyzed with the model presented here and the effect law of different factors on the bearing capacity of the structures in fire are discussed. It is concluded that fire response of the pre-stressed steel trusses is affected obviously by the different model of material under the same fire conditions, and critical temperature of the structure in fire increases with strength grade of steel material, which means that choice of the proper material model used for analysis of behavior of the steel structure under fire is very important and need to be studied further.