Hi! What is the temperature in Beijing on Mid October? We are going there October 16-20...

Generally, the temperature may range from 8C to 20C, a little bit cold, so the jackets may be necessary for daytime and night.

Autumn in BJ is very short. And weather in October is fairly unpredictable. One day you could be westing summer clothing. The next day the temperature drops drastically. Wear pants and long sleeved top. Expect to layer your clothing as when needed hot / cold.
If winter comes early - expect to put on thick jacket.