I am planning to hongkong in August and September. how will be the weather?

It can be hot and wet during that time. The average temp may reach around 30C. The rainfall is quite dense. I suggest you wear summer clothing and prepare simple rain gear in case. Sun cream as well as sunglasses are also needed. In addition, it will be during typhoon season, but it won't severely happen to my experience. Anyway, just pay attention to the weather forecast!