There　are　a　few　measured　data　of　the　residual　stresses　owing　to　its　difficulty　and　high-cost　to　measure　the　autofrettaged　residual　stress.　The　grey　prediction　method　is　introduced　in　this　study　to　find　the　relaxation　law　of　autofrettaged　residual　stresses　in　stable　temperature　field　and　under　pressure　fluctuation,　and　grey　relational　analysis　is　applied　to　find　the　influence　of　temperature　and　pressure　fluctuation　in　ultra　high-pressure　polyethylene　reactor　on　the　relaxation　of　residual　stress.　The　results　of　grey　treatment　are　consistent　with　the　experiments.　It　thus　can　be　seen　that　the　grey　prediction　method　provides　a　valid　and　convenient　way　for　research　on　the　relaxation　of　autofrettaged　residual　stresses.　and　that　it　has　practical　values　in　the　integrity　assessment,　design　and　maintenances　of　autofrettaged　vessels.