The fatigue crack propagation rate (da/dN) of bimodal structure Ti55531 titanium alloy before and after laser shock peening（LSP）was investigated. The fracture, microstructure and residual stress of fatigue crack propagation samples were analyzed. The results show that after laser shock, the fatigue crack growth rate da/dN decreased. When △K < 22.84MPa?√m, the sample BM-LSP has a lower fatigue crack growth rate than the sample without laser shock BM. When △K=22.84 the crack growth rates of the two samples were similar that is 3.92×10-4mm/cycle. After LSP, the length dispersion and thickness dispersion of the secondary α layer decreased by 22.9%, 38.9%, and the polar density of α and β phases decreased by 37% and 16%, respectively. The passivity of the lamer α tip and microstructure homogenization alleviated the stress concentration, resulting in a decrease in da/dN. In addition, the laser shock process introduces a residual compressive stress layer to a depth of about 900μm on the surface of the material. Residual compressive stress is also an important factor to offset tensile stress at crack tip, enhance crack closure and slow down crack propagation.