The　causes　of　frequent　haze　in　Beijing-Tianjin-Hebei　were　truly　complex,　but　the　researches　on　the　interrelation　between　haze　and　socio-economic　factors　were　still　lack　of　quantitative　analysis.　Considering　that　the　interrelation　under　different　regimes　showed　different　change　rules,　the　modified　panel　threshold　regression　model　was　applied　to　address　different　policy　challenges.　The　results　showed　that　the　single　threshold　effect　was　significant　while　the　added　value　of　the　tertiary　industry　or　public　transport　was　selected　as　the　threshold　variable.　When　the　added　value　of　the　tertiary　industry　was　less　than　the　threshold　value,　the　development　of　the　secondary　industry　and　the　rising　number　of　cars　would　increase　PM2.5.　Therefore,　the　expansion　of　the　public　transport　would　be　an　effective　measure　to　ease　the　haze.　When　the　added　value　of　the　tertiary　industry　exceeded　the　threshold　value,　the　continued　development　of　the　secondary　and　the　tertiary　industry　would　reduce　PM2.5,　but　the　relief　function　achieved　by　expanding　the　public　transport　would　be　weakened　slightly.　Meanwhile　the　number　of　vehicles　would　become　the　primary　factor　influencing　PM2.5.　When　the　number　of　public　transport　was　closer　to　its　threshold　value,　the　opportunity　cost　of　PM2.5　reduction　would　be　less　and　the　increase　of　energy　consumption　would　always　increase　PM2.5.　As　there　were　nonlinear　relations　between　the　haze　and　the　socio-economic　factors,　the　policy　formulation　of　PM2.5-reduction　should　be　combined　with　the　threshold　effect.　©　2017,　Editorial　Board　of　China　Environmental　Science.　All　right　reserved.