Reasonable determination of construction water content is of great significance to ensure the best comprehensive performance of subgrade filling. In order to investigate the influence of water content on the strength deformation characteristics of fly ash subgrade and determine the water content suitable for the construction of fly ash subgrade, the fly ash in Ningxia area was used as the test material. Combined with the results of compaction test, consolidation compression and shear were carried out to analyze the mechanical properties of fly ash under different water content, and the best water content under each single factor was determined. Based on the gray relational analysis, the construction water content of fly ash subgrade can be proposed to ensure the best comprehensive performance. The results show that with the increase of water content, the coefficient of compression decreases first and then increases, the modulus of compression increases first and then decreases, the final vertical strain fluctuates within a certain range, and the compressibility of fly ash is the smallest when the water content is 31%. At high normal stress and high water content, the shear stress-shear displacement curve of fly ash shows strong strain softening type. With the increase of water content, the residual internal friction angle decreases, the residual cohesion increases and then decreases and tends to stabilize, and the residual shear strength of fly ash is the largest when the water content is 23%. When the water content in the range of 31%~35% of the mechanical index correlation degree value maintained in a high and relatively stable interval, which can be used as a guide to the construction of the comprehensive water content.