Aiming at the characteristics that the bolt stretcher for conventional island of nuclear power has the ultra high pressure, ultra small space operation conditions, a design method based on numerical analysis and experiment was presented, the ANSYS software was used to check the cylinder block and blind nut, and the analysis results were compared with the experimental strain data. The optimization characteristices of the parameters Rm, D on the blind nut were analyzed. The results show that : the strength and stiffness of first set of cylinder block can meet the material requirements, but the blind nut has “negative” safety factor in the inflection point of “L” section; optimizing the blind nut structure size, when Rm and D are equal to 8 mm and 60 mm respectively, the stress decreases by 121.2% and the strain decreases by 58% comparing with initial parameters, the safety coefficient is heightened, which provides design basis for the reliability and safety of the equipment.