My husband and I go first to Beijing then visit to Kunming in September. What clothes do we need?

During Sep. Beijing is a good place for traveling with mild temperature and plenty of sunshine, and the average temperature could be about 20 C. So long-sleeved shirt and thin coat are ok for you. Besides, it would rain a lot, so the rainy wear or an umbrella may help you a lot then.
For Kunming, Sep. is still during the rainy season. The temperature may drop quickly along with the rain, so take a raincoat and a light overcoat is suitable then.