I want to fly from changsha,china to Dhaka,Bangladesh, do i need the transit visa for Hongkong

Both of you need transit visas even when not leaving the airport unless you hold diplomatic or official passports. The Hong Kong Immigration Department's page on visit/transit visa has detailed requirements. However it's generally not possible to get a visa in under 4 weeks unless you know someone who is currently in HK and can submit the application for you.