Abstract: To study the over conservative margin of shock stress reduction factor in stainless steel piping designed by elasto-plastic rules, a method is used by carrying out elastic and elasto-plastic calculations for stainless steel piping with scaled three kinds of independent loads of gravity, pressure and shock loads, respectively, to obtain nominal stress and actual total strain for manifesting shock load’s less damage effect on stainless steel piping. Comparative analysis results show that shock stress reduction factor 0.25 stipulated in shock codes is of conservative margin, and a new value of 0.2 is proposed for elasto-plastic design. Conclusion is that a new shock stress reduction factor of 0.2 by elasto-plastic design can not only meet the code requirements of conservatism but also reduce the over conservative margin in shock stress evaluation.