The porous characteristics of porous materials result in a distinct fatigue crack extension mechanism under cyclic stress compared to monolithic materials. The study employs 18Ni300 mold steel to fabricate porous CT specimens with varying pore diameters, depths, and spacings using a selective laser melting technique with 18Ni300 powders. Pore morphology was a single variable, and fatigue crack extension tests were conducted under different loads and stress ratios. The Paris formula was applied to fit test data, and crack extension paths were simulated and predicted using the extended finite element method. The study reveals that larger pore diameters and depths increase crack extension rates and reduce life, while smaller spacings lead to lower rates and longer life. Pores also affect the material’s da/ dN–∆ K curve, with higher depth causing larger sudden change amplitude. Pores also influence crack extension trajectory, resulting in a more linear crack propagation.