The effects of multistep shot peening on the structure and mechanical properties of 18CrNiMo7-6 steel have been investigated using X-ray diffraction line profile analysis. X-ray diffraction data reveals the phase transformation from austenite to martensitic phase after SP treatments. The results show that multistep shot peening can more significantly improve the mechanical properties. The optimal peening intensity of 0.50+0.30+0.15mmA is found for 18CrNiMo7-6 steel. With the optimized shot peening treatment, the maximum compressive residual stress (located at the depth of 20μm for martensite) and top-surface compressive residual stress of martensite attain the values of 1463 and 1256MPa, respectively; While those of austenite are 1039 and 766MPa, respectively. The domain size of 18CrNiMo7-6 steel decreases to 11.1nm on the top surface after shot peening with the intensity of 0.50+0.30+0.15mmA, and the micro-hardness on the top surface attains the peak value of 1175 HV.