bus to Niu Jie Mosque from TianAnMen Square

You can take bus no.5 at West Square Station, get off at West Niu Jie Crossing Station. Then walk south along Niu Jie around 550 yards (500 meters) to find the mosque.

do u know the closest mosque to Tiananmen square or forbidden City?

The nearest one is Qianmen Mosque. It's located at Yangwei Hutong, Langfang Ertiao, Xicheng District.

Thank U Mia. Could u pls describe the direction how to get there?

Walk south along Qianmen Avenue from Qianmen Subway Station for about 300 yards, and turn right. Next, walk west along Langfang Ertiao Street for about 300 yards, and you will reach Qudeng Hutong. Then, walk west along Qudeng Hutong for about 200 yards, and you will see it.