The double-ring infiltrometer was the most used and classic field method to measure soil water infiltration process and saturated hydraulic conductivity, and the sizes of inner and outer rings of the infiltrometer would affect the determination result. Seven groups of double-ring infiltrometers, with different inner and outer diameters, were laid out and conducted in the grassland flat field in the Heihe river upstream in Qilian mountains, to investigate effects of the size of the infiltrometers on the measurement of the infiltration process and saturated hydraulic conductivity. The results showed that if the inner diameters of double-ring infiltrometer were same, the larger the outer diameter, the smaller the stable infiltration rate and saturated hydraulic conductivity; and if the outer diameters were same, the smaller the inner diameter, the smaller the stable infiltration rate and saturated hydraulic conductivity. The stable infiltration rate and saturated hydraulic conductivity were significantly negative correlation with the buffer index of double-ring infiltrometers, but there was no clear correlation between the initial infiltration rate and the buffer index. Considering the simplicity of installation, water demand and apparatus accuracy, the double-ring infiltrometer with the diameters of 20 cm and 40 cm for the inner and outer rings was recommended to measure the soil water infiltration process and saturated hydraulic conductivity in the mountainous regions.