I will travel to Beijing on 27-09 to 03-10, what is the weather ? What should I have to bring?

Generally, it is one of the best time to visit the city, mostly sunny and either too hot nor too cold. You can bring long-sleeve t-shirt and a thin coat.
BTW, Oct-1-7 is the national day holiday in China and there will be crowds in tourist sites and transport tools; be prepared.