[Objective] Research on the county-scale spatial variability of soil available nitrogen(AN) and determination of reasonable sampling density in order to provide scientific knowledge for nutrient management in the Qinba mountain areas.[Methods] Sampling schemes with different sample intensities were simulated by extracting data from an AN database of intensive samples, and then made the spatial interpolation analysis of AN based on geostatic combined on GIS technology, and the interpolation accuracy was evaluated by the cross validation method.[Results] (1) The content of AN showed moderate variation with the coefficient of 42.95%; (2) The moderate spatial variability of the AN was found with the ratio of Nugget and Sill of about 1/2 and the maximum spatial self-related distance of 9 171 m; (3) Significant changes were detected when sample number reduced from 1 060 to 742, meanwhile, great changes were found of variation distance and the ratio of Nugget and Sill with the relative errors of 152.32% and 36.1%, and the root mean square error (RMSE) and relative coefficient changed significantly.[Conclusion] The range of the AN in Hanbin District was longer than other regions, so spatial continuity was better. It is possible to reduce the number of sample points to satisfy the need of the research regarding spatial variability of AN. Taking the results of spatial prediction accuracy into consideration, more than 1 060 samples should be the suitable number to evaluate the spatial variability of AN on county scale, and the maximum sampling unit is 345.5 hm².