In　order　to　simulate　the　diffusion　of　toxic　gas　in　a　timely　and　effective　way,　based　on　the　PISO　(pressure　implicit　with　splitting　of　operators)　algorithm,　the　simulation　of　gas　diffusion　in　urban　areas　with　OpenFOAM　software　was　carried　out.　A　comparison　was　conducted　between　the　SIMPLE　(semi-implicit　method　for　pressure　linked　equations)　algorithm　and　PISO　algorithm.　The　PISO　algorithm　has　priority　in　terms　of　tolerance　and　stability　for　simulation　of　wide-range　gas　diffusion　in　urban　areas　with　neighboring　diffusion　zones.　With　OpenFOAM　software,　the　diffusion　and　spreading　process　of　chlorine　in　Yishan　Campus　of　Fuzhou　University　was　simulated.　The　simulated　results　by　FLUEN　software　were　then　compared　under　the　same　conditions.　The　results　showed　that　OpenFOAM　could　simulate　the　diffusion　process　of　poisonous　gas　under　the　complicated　virtual　geographical　environment.　The　mean　deviation　of　the　simulated　result　by　OpenFOAM　was　5%　less　than　that　by　the　FLUENT;　however,　the　computational　efficiency　was　improved　by　41.89%.　©,　2015,　Editorial　department　of　Molecular　Catalysis.　All　right　reserved.