<p>Room-temperature low-cycle fatigue (LCF) tests were performed on the novel 17-4PH steel used as steam turbine last-stage blades at total strain amplitudes (Δ<i>ε</i>_<i>t</i>/2) ranging from 0.6 to 1.3%. Fatigue life (<i>N</i>_<i>f</i>) prediction of the steel was achieved in terms of the Manson-Coffin and Smith-Watson-Topper (SWT) models, respectively. Furthermore, the effect of notches on LCF performance of the steel was also investigated concerning water erosion effect of the last-stage blades. The results show that the steel exhibits pronounced cyclic softening, tensile–compressive asymmetry, and mean stress relaxation characteristics during LCF. Moreover, on the LCF fracture surfaces of the steel, the crack initiation zone, crack propagation zone and final fracture zone can be clearly identified. Especially for the crack propagation zone, an evident feature of fatigue striations and secondary cracks is presented. When Δ<i>ε</i>_<i>t</i>/2 ≥ 1.0%, the SWT model can more accurately predict <i>N</i>_<i>f</i> of the steel; however, when Δ<i>ε</i>_<i>t</i>/2 ≤ 0.8%, the Manson-Coffin model is more suitable. As for the notched specimen, the circumferential notch root actually acts as “deterministic crack initiation zone”, resulting in formation of the concentric-ring-like crack propagation zone and disk-shape final fracture zone. The <i>N</i>_<i>f</i> of them is extremely shortened. Therefore, the water erosion pits result in a significant reduction of service life of the 17-4PH steel last-stage blades in deep peak-shaving operation.</p>