Phosphorus can increase the creep and stress-rupture properties of the turbine disk materials served in high stress with short life. However, the effect of P on the creep of boiler tube in advanced ultra-supercritical power plants served in low stress with long life is still unknown. Combining the formula derivation and experimental analysis, the varying effects of P on the creep with the decrease of stress were clarified. The positive effect of P on prolonging the creep rupture life reduces with the decrease of creep stress, even vanishes when the creep stress decreases to 93 MPa, 133 MPa and 140 MPa in the alloy doping 100 ppm, 200 ppm and 400 ppm P, respectively. The degeneration of M23C6 and the dynamic segregation of P at grain boundary during long-term creep under low stress can decrease the deformation coordination between the adjacent grains and the grain boundary binding force, which makes the positive effect of phosphorus on the creep diminish.