In　order　to　improve　mechanics　performance　of　the　internal　stretch　forming　roof　truss　(ISFRT),　twenty　computation　models　with　different　sag-span　ratios　and　rise-span　ratios　were　built.　The　influence　of　sag-span　ratio　and　rise-span　ratio　on　mechanics　performance　was　analyzed　using　parametric　design　language　APDL　programming　analysis.　Study　shows　that　with　the　increase　of　rise-span　ratio　and　the　decrease　of　sag-span　ratio　of　ISFRT,　the　axial　force,　the　bending　moment　and　the　cross-section　stress　of　upper　chord　middle　elements　in　the　forming　process　increase　gradually,　and　the　ISFRT　state　in　the　loading　process　is　formed　by　subjoining　special　axial　force　to　the　state　in　the　forming　process.　Additionally,　bending　strain　is　the　main　mechanics　performance　in　the　forming　stage,　in　which　cross-section　tension　stress　is　close　to　compressive　stress,　so　the　axial　force　of　ISFRT　is　small.　And　the　increase　in　axial　compressive　strain　is　the　main　mechanics　performance　in　the　loading　stage,　so　the　axial　force　of　ISFRT　is　great.　Selecting　appropriate　sag-span　ratio　and　rise-span　ratio　can　improve　mechanics　performance　of　ISFRT.