Abstract: Using #425 Portland cement, fly ash and UEA expansive agent to develop fly ash based UEA modified grouting material, the shrinkage measurement and uniaxial compressive strength mechanical experiment of 15 grouts with different ratios during curing period were carried out in the laboratory by rigid testing machine to study the natural deformation, mechanical properties and instability mode of fly ash based UEA modified grouting material. The uniaxial compression test is carried out, the influence of UEA content on the shrinkage, failure strength, peak strain and instability mode of grouts is analyzed, and the instability mode and critical value point of fly ash based UEA modified grouts are determined. The results showed that with the increase of UEA content gradient (0 ‰, 2 ‰, 4 ‰, 6 ‰, 8 ‰), the initial shrinkage of stone body decreased by 41.55%, 2.63%, 1.77% and 2.22% respectively, and the later shrinkage of grouts decreased by 45.61% and 61.29% respectively (C0, C1 and C2); With the change of UEA content gradient, the failure strength of the sample increased by 49.15%, increased by 3.96%, decreased by 18.18% and increased by 4.95% respectively; With the change of UEA content gradient, the peak strain of stone body increased by 1.66%, 1.41%, 4.37% and 1.70% respectively. UEA expansive agent significantly increased the peak strain of grouts, showing a linear increasing trend; The failure and instability modes of fly ash based UEA modified grouts are low-strength large deformation complex failure and high-strength small deformation failure. The critical point of the two instability modes is that the failure strength of slurry stone body is 1.5MPa.