A night-sky luminance survey was carried on in Beijing to assess the level of light pollution. The luminance of the zenith night sky and skies in four directions at six sites with different distances from the city center was measured by using a photometric luminance meter. The Xinglong Station of National Astronomical Observatory was included to study the impacts of city lights on an astronomical observatory. The survey shows that the night-sky luminance decreases with increasing distance from the city center. Measurement results indicate that outdoor lighting in the Xinglong county town which is close to the observatory has non-negligible influence on the night sky at Xinglong Station.