Much attention has been given to the distributions, sources, and health risks of atmospheric polycyclic aromatic hydrocarbons (PAHs) in cities. In this study, a total of 62 suspended particle samples were collected from April 2013 till March 2014 in the city of Lhasa. Positive matrix factorization (PMF) was applied to investigate the source apportionment of 15 priority PAHs, the lifetime carcinogenic risk (LCR) levels of which were assessed. The average annual particle phase PAH concentration was 43.9 ± 60.4 ng m–3. Evident seasonal variations of PAHs were observed, with the highest concentration observed in winter, followed by autumn, spring, and summer. Four- and five-ring PAHs accounted for the predominant proportion (63.3%–84.4%) throughout the year. Correspondingly, gas phase PAHs showed the opposite variations, with the highest and lowest concentrations observed in summer and winter, respectively; also, three-ring PAHs, especially Ace, Acel, and Flu, were the largest contributors. Compositions of particle phase PAHs varied seasonally, with four-ring PAHs contributing more in winter than in summer and five-ring PAHs exhibiting the opposite trend, thereby reflecting the variety of emission sources. PMF analysis showed that biomass combustion (48.4%) and vehicle emissions (27.9%) were the two main sources, followed by coal combustion and the air–surface exchange. These results were consistent with the diagnostic molecular ratios. The benzo(a)pyrene equivalent (BaPeq) concentration of particle phase PAHs ranged from 1.48 to 24.5 ng m–3, which exceeds were higher than the new limit in China (1 ng m–3). The average BaPeq of gas phase PAHs was 6.43 ± 4.15 ng m–3, which was similar to that of particle phase PAHs. The LCR of the total PAHs (9.08 × 10–6) was one time higher than that of the particle phase; however, it was slightly lower than the acceptable level, thereby indicating that atmospheric PAHs in Lhasa pose little or no carcinogenic risk to the local population.