Biomass burning (BB) in Southeast Asia is particularly pronounced during the dry season. However, the complex topography and long-range transport inherent to Southeast Asia have limited local research on pollution resulting from BB. In this study, the monthly variation in aerosol optical properties at six sites in Southeast Asia (Chiang Mai, Mukdahan, Bac-Lieu, Penang, Singapore, and Bandung) and the fire-point distribution have been analyzed in detail. The Hybrid Single Particle Lagrangian Integrated Trajectory (HYSPLIT) model was used to simulate the 72-hour back-trajectory from the Shangri-La atmospheric background station in Yunnan Province, China. Our results showed that BB was more common on the Indochinese Peninsula from March to May, whereas it was more common on the Malay Archipelago from August to October due to the latitudinal difference and crop harvest season. Significant BB activity on the Indochinese Peninsula in March resulted in a high surge in extinction (AODt440nm = 1.32 ± 0.69, AODf440nm = 1.24 ± 0.59) by particles with a smaller diameter (AE = 1.68 ± 0.13) in Chiang Mai. Mapping the long-range transport of BB aerosols reveals that Shangri-La’s pollution was primarily affected by emissions from northern-central India (accounting for 45.2%), and Bangladesh and northern Myanmar (accounting for 38.7%), which indicates that the aerosol pollution on the Yunnan-Guizhou Plateau in springtime could have originated on the southern periphery of the Tibetan Plateau. The results also indicate that BB emission in Southeast Asia had a limited impact on pollution in Southwest China but a relatively large effect on local areas. This study is the first to analyze the trend of aerosols produced from BB in Southeast Asia via ground-based observation, which deepens our understanding of the potential effects of BB aerosols transported long-range from outside Southwest China.