The Tibetan Plateau is regarded as a typically sparsely populated area both from Chinese and global perspectives. There exists a large difference in population distribution of the Tibetan Plateau. Based on the town-level census data in 2010, we compiled a spatial population database to calculate the population density of the Tibetan Plateau. We found that the population distribution on this plateau was characterized in four dimensions, namely longitude, latitude, polarization and altitude. In addition, similar to the "Hu Line" for Chinese population distribution, we introduce the "Qilian-Gyirong Line" (short for Qi-Ji Line), a spatial division line of population geography in the Tibetan Plateau. Our main results are summarized as follows. (1) There are significant regional differences in the spatial population distribution in the Tibetan Plateau. The population density in the southeast was higher than that in the northwest. Compared to longitude-oriented, latitude-oriented and altitude-oriented regional differences, the polarization-oriented regional difference of population distribution was more prominent. This finding indicates that fewer people live in areas close to the cold-arid core of the Tibetan Plateau. (2) The spatial population distribution is closely related to the natural and environmental factors, including altitude, soil, water resources, climate and vegetation. The interaction between the natural factors has an impact on enhancing the regional difference of population. (3) The "Qiji Line" reveals the spatial differences of population in the Tibetan Plateau. The areas on either side of the line are approximately similar, whereas the population shares in the southeast and northwest halves are 93% and 7%, respectively. (4) In general, the population differences on both sides of the "Qiji Line" were stable over time. During 1982-2010 the population share in the southeast half decreased counterbalanced by an increased share in the northeast half. In the future, the majority of the people are expected to live on the southeast side. This is likely to imply two major policy initiatives. More attention is required on the urbanization development and transportation network construction in the southeast half and the increasing eco-environmental pressures from the growing population in the northwest half.