Abstract: Using the vertical component records of 150 broadband seismic stations from January to December 2010 in the southeastern Gansu region, this paper calculated the ambient noise cross-correlation functions between each station-pair and stacked them to obtain Rayleigh surface wave signals in the periods of 5-10 s and 10-20 s. And then the method of signal-to-noise ratio (SNR) and normalized background energy flow (NBEF) measurement are adopted to study the characteristics of ambient noise source in southeastern Gansu region based on the asymmetrical features of cross-correlation functions. The results show that the seasonal variations of ambient noise source for both periods of 5-10 s and 10-20 s have their respective azimuthal distribution of noise energy. The azimuth of ambient noise source in the period of 5-10 s is 170°-240° in summer, which means the energy of ambient noise source is mainly affected by the Indian Ocean, and it moves to 100°-150° in winter, which means the energy of ambient noise source is mainly affected by the North Pacific Ocean. In contrast, the characteristics of ambient noise source in the period of 10-20 s is more complex, because the azimuth of ambient noise source is controlled by several oceans. The energy of ambient noise source focuses on the azimuth of 170°-210° in summer, which means it is mainly affected by the Indian Ocean, then it moves to 90°-150° and 310°-355° in winter, which means it is mainly affected by the North Pacific Ocean and the North Atlantic Ocean. Because the ambient noise source of southeastern Gansu region has obvious seasonal variation in both periods of 5-10 s and 10-20 s, inhomogeneous ambient noise source should be taken into consideration when the method of ambient noise is used to study the seismic velocity structure in the region.