Office space agglomeration problem is one of the main topics in urban geography research. Office cluster, as an important form of industry cluster, reflects a characteristic of new economic growth space after urban economies transformation. Furthermore, there is an inevitable outcome of urban development to top-ranked level. On the basis of the locations of 976 office buildings in Beijing city including Beijing Economic-Technological Development Area (Yizhuang), this paper uses the kernel method firstly to analyze the respective concentrative degree of the spatial distribution of urban office activities. Then, with the aid of Crimestat software, it distinguishes different order clusters, investigates the present situation of office cluster, and demonstrates its distributional features and the formation causes. The results show that there are obvious office clusters in Beijing city, and three features are identified. The first is that there is remarkable difference between south and north with the Chang'an Avenue as the divide. The first order clusters are mostly situated in the north, and there is also difference between east and west separated by the traditional central axis, in particular they tend to be concentrated east of the axis, presenting a dispersal pattern on the whole with partial concentration. The second is that the first order clusters come along with transportation lines and the direction of long axis parallel lines. The third is that nearly 3/4 of office clusters are concentrated in urban functional expanding area, consisting of four districts in outer city, and that only 1/4 of office clusters are concentrated in inner city, which is different from office activities agglomeration in inner city in the western metropolises. The reasons can be summarized as follows. First, economic function division has promoted the formation of office clusters. Second, the boundaries of administration districts have become the natural obstacles to the expansion of office space. Third, the distribution of office clusters has been greatly affected by urban function. So, CBD, financial street, Zhongguancun and Shangdi are the representative office clusters in Beijing city.