Cosmic-ray Soil Moisture Observing System (COSMOS) is a newly-developed cosmic-ray instrument for measuring average surface soil moisture content over an area with more than 600 m in diameter. A field test was conducted of using this instrument to measure surface soil moisture content in a tract of grassland in the Loess Plateau, and comparison was made of the results of the field test with the results of the oven-drying method, and of the point scale measurement with the HydraProbe II sensors. It was found that the measurements using the COSMOS method were well in agreement with those using the oven-drying method, but differed from those using HydraProbe II sensors in some time spans, suggesting that soil moisture content in the Loess Plateau was subject to spatio-temporal variability. The COSMOS instrument offers an effective approach to measurement of area-average surface soil moisture content in the Loess Plateau with fragmented surface, and a potential tool for studying spatial variability of soil moisture content in the Loess Plateau.