In order to explore the effects of different proportions of fly ash composited with aeolian sandy soil on the fractal dimension and fertility of sand particles under drought stress and to provide a theoretical basis for fertility promotion of Mu Us sandy land, four fly ash levels (F0, F1, F2, and F3, accounting for 0, 1%, 5% and 10% of dry weight of aeolian sandy soil, respectively) and three water levels[W1, W2 and W3, accounting for (75±5)%, (55±5)% and (35±5)% of the field water capacity, respectively] were set up in the experiments, and a pot cultivation method was used for water control. The fractal dimension was used to reflect the particle size distribution of aeolian sandy soil under the action of water and fly ash. And the principle component analysis and hierarchical cluster analysis method were used to evaluate the fertility change of aeolian sandy soil under different treatments. The results showed that:(1) Under the same water condition, the fractal dimension of aeolian sandy soil increased with the increasing of fly ash proportion. (2) Under the same proportion of fly ash, the content of nitrate nitrogen (NO₃^--N) of aeolian sandy soil showed an increasing trend with the improvement of drought stress utilization efficiency. (3) The total nitrogen (TN) content of aeolian sandy soil was at low level under treatments of fly ash and drought stress. Nitrogen fertilizer application should be considered in the actual production process. (4) The comprehensive evaluation results of aeolian sandy soil showed that 10% composite ratio of 10% fly ash had good effect and it had certain applicability in drought situations.