Where is the nearest Carrefour to the marriott imperial mansion hotel in Dongcheng.

Well, the nearest Carrefour is at Fangchengyuan, Fangzhuang, Fangcheng district. It's about 3.5 miles away from your hotel.
The nearby subway stations to your hotel are Wangfujing on line 1 and Dongdan on line 5, while the closest one to that Carrefour is Puhuangyu on line 5.
From your hotel to the supemarket, you can walk to Dongdan to take line 5, get off after 4 stops, and then walk there.