Abstract: The cracked round bar (CRB) method was used to test the fatigue crack growth (FCG) behavior of PE100 pipe butt-fusion joint under different load ratio. The fatigue crack propagation mechanism of PE100 pipe butt-fusion joint was discussed. The slow crack growth (SCG) resistance in static loading condition was obtained by extrapolation. The results show that when load ratio R was the same, the fatigue crack propagation rate of PE100 pipe butt-fusion joint increased and the fatigue life shortened with the increase of maximum initial stress intensity factor K_max. When K_max was the same, the fatigue crack propagation rate decreased and the fatigue life extended with the increase of R. The stretched length of crazing fibrils in the crack discontinuous growth region reduced and the secondary cracks overlaid the main crack in the crack stable growth region which accelerated the crack propagation rate of butt-fusion joint. The allowable maximum initial stress intensity factor was 0.555 MPa·m^1/2 for the service life of 50 years under static loading condition calculated by extrapolation.