Why do you use though? Though means 'despite the fact' or same with 'although'. Since your statements are not contradicting, using though here is not so suitable. Maybe your sentence should be - Baoan's image is still the “big industrial district” because its GDP always ranks top among the other cities in Shenzen. Or Because its GDP always ranks top among the other cities in Shenzen, Baoan's image is still the “big industrial district”. Hope that helps.