Based　on　the　system　analysis　method,　a　constitutive　equation　for　concrete　was　put　forward.　The　parameter　of　fatigue　damage　of　concrete　was　defined　in　this　paper.　Relationships　between　the　fatigue　damage　parameter　and　the　detection　of　fatigue　damage　in　the　concrete　mixed　up　with　different　contents　of　flyash　were　discussed.　The　influence　of　flyash　on　crack　process　of　the　concrete　is　discussed.　The　process　of　the　fatigue　damage　in　the　concrete　is　obviously　different　and　the　fracture　process　is　the　continuous　growth　of　cracks　in　the　interfacial　transition　zone　(ITZ)　between　mortar　and　aggregate　when　the　concrete　was　mixed　up　with　more　than　30%　flyash　after　a　curing　age　of　60　days,　whereas,　various　intensity　of　flyash　concrete　reduced　less　with　the　increase　of　flyash　content.