Aim at locomotive axle of HXD1 and HXD2 high-power heavy-load freight locomotive applied in Da-Qing railway turning into fatigue period and generating fatigue crack, axle test module was produced, and probe angle for small-angle crack detector was calculated. Method of using straight probe on axle-end surface and scanning crack distribution region with small-angle probe was realized, and problems should be concerned in practical operation were pointed out. Results showed that the method could detect fatigue crack of locomotive axle effectively.