Debris flow is one of geological disasters in Mountains. The low frequency debris flow bring serious threaten to building and life safety in mountain areas. In mountain areas of southwest China 7 debris flow gullies were investigated and particle sizes were measured. The coarse particles size of long axes，middle axes and short axes were measured in low frequency debris flow gullies. From the analysis of measure result, the characteristic parameters of low frequency debris flow in mountain areas of southwest china were obtained: the coarse particles average size of long axes、 middle axes and short axes are 52,36,21 cm. The coarse particles number percentage of long axes more to 50 cm between 36% to 58% and average number percentage are 50%. The linear density of coarse particle long axes more to 50 cm between 0.43 to 1.10 per meter and average linear density is 0.84 per meter. The research result provides basic data to early identification system of low frequency debris flow in mountain areas of southwest china.