In　the　three　years,　50%　of　the　fluctuating　vacuum　sterilizer　made　in　China,　cracked　on　their　inner　surfaces　during　service.　The　stress　of　the　sterilizer　under　load　is　measured　and　analyzed　to　find　out　the　cause　of　cracking.　Static　and　dynamic　loads　are　applied　to　a　sample　sterilizer,　whose　working　stress　is　electronically　measured　with　a　specially　test　device.　Experimental　results　show　that　the　inner　surface　stress　under　working　load　is　quite　low,　the　maximum　Mises　stress　at　the　weld　heat　affected　area　is　only　0.21s,　the　static　and　dynamic　working　loads　are　equivalent,　and　no　pulse　load　is　found　with　the　working　pressure　fluctuation.　To　evaluate　the　weld　residual　stress　of　a　sterilizer,　four　weld　plates　with　different　welding　process　are　measured　with　the　X-Ray　method.　Experiment　shows　that　the　residual　stress　is　as　high　as　0.89s　at　the　heat　affected　area.　The　combination　of　the　high　residual　stress　and　the　normal　working　stress　made　the　sterilizer　inside　surface　stress　exceeded　the　yield　stress　of　the　sterilizer　material　at　the　weld　heat　affected　area.　Under　the　fluctuating　and　cycling　stress　and　corrosion　working　condition,　the　high　residual　stress　at　the　weld　heat　affected　area　is　the　main　cause　of　the　occurrence　and　expansion　of　cracks.