There are many time-honored halal restaurants in Beijing, so that you could find one easily, such as Qianmen Street and Wangfujing Street. As for a whole list, it is suggested that you read this website's introduction on Beijing dining.

Hong Bin Lou
Chinese name: 鸿宾楼（hng bīn lu）
Location: No.11, Zhanlan Lu, Xicheng District
Bus Route: 15, 814, 709, 708 (get off at Baiwanzhuang Dongkou Stop)

Nanlaishun Restaurant
Chinese name: 南来顺饭庄（nn li shn fn zhuāng）
Location: No.12, Nancaiyuan Jie, Xuanwu District
Bus Route: 59, 102, 122, 819 (get off at Daguanyuan Stop)

Xinjiang Restaurant
Chinese name: 新疆饭庄（xīn jiāng fn zhuāng）
Location: No.1, Chegongzhuang Dajie, Beili, Xicheng District

For more information, you can look for this website：https://www.travelchinaguide.com/cityguides/beijing/dinning/muslim.htm