Dear CJ Tung,
Greetings from Top China Travel! Thanks for your message.
There is a restaurant called Fangshan (focus on imperial cuisine) inside the Beihai Park - the East Gate. So you have to get in the park first.
But generally speaking, this restaurant caters mainly for tourists in group, for individual travelers, it only offers set meal.
If you have any further question, please feel free to contact us.