The　position　of　a　macrocrack　within　a　circular　tube　can　be　determined　through　the　periodic　energy　distribution　variation　of　a　linear　non-axisymmetric　guided　wave.　However,　to　determine　the　positions　of　a　microcrack　within　a　circular　tube　is　still　a　difficult　job,　even　for　the　nonlinear　guided　wave.　Unlike　the　linear　guided　wave　can　detect　the　axial　position　through　the　reflection　from　a　macrocrack,　a　fatigue　crack　does　not　even　cause　remarkable　reflection.　This　paper　investigates　the　energy　concentration　degree　periodical　variation　rule　of　fatigue　crack-　induced　second　harmonic　non-axisymmetric　guided　wave　within　a　circular　tube.　The　quantitative　relationship　between　the　energy　concentration　degree　and　its　propagated　axial　distance　is　potential　to　be　used　to　detect　a　fatigue　crack　and　characterize　its　axial,　circumferential　positions.　To　facilitate　the　investigation,　a　specific　frequency　extracting　method　was　optimized　to　extract　the　second　harmonic　non-axisymmetric　guided　wave.　In　addition,　an　energy　concentration　degree　coefficient　was　designed　to　characterize　the　energy　distribution.　The　fundamental,　numerical　and　experimental　work　were　implemented　to　verify　the　studied　energy　concentration　degree　periodical　variation　rule.