The　temperature　distribution　is　a　key　to　analyzing　fire　behavior　in　large　space　steel　structures.　However,　most　available　calculational　methods　are　rather　complicated　and　not　suitable　for　design　analyses.　This　paper　presents　a　simplified　method　based　on　the　assumption　that　the　temperature　ratios　in　the　structure　components　are　the　same　as　the　air　temperature　ratios　at　the　same　locations　during　the　fire　temperature　increase　phase.　The　assumption　is　also　extended　to　the　cooling　phase　to　develop　the　simplified　calculational　method　for　the　temperature　field　in　the　structure　during　the　temperature　increase　and　cooling　phases.　Numerical　simulations　for　a　lattice　grid　structure　using　the　simplified　method　agree　well　with　predictions　using　the　conventional　precise　approach.　Thus,　this　simplified　method　can　be　used　to　predict　the　fire　behavior　in　large　space　steel　structures　during　the　design　phase.