<p>To overcome the issue of compressive buckling instability and the inconvenience of strain extensometer installation in conventional strain-controlled low-cycle fatigue tests using small specimens, a plate-shaped small specimen enabling stable strain control via side-span strain was designed based on elasto-plastic finite element deformation analysis, and its optimal dimensions were determined. Based on the equivalent energy density principle, a semi-analytical true stress–strain relationship of plate-shaped small specimens was proposed, which enabled an effective conversion between the side-span-controlled nominal strain and the true strain at the specimen center. Based on this, a new low-cycle fatigue testing method for plate-shaped small specimens was established. Low-cycle fatigue tests on 6061 aluminum alloy at room temperature were conducted using both the small specimens and standard cylindrical specimens. The results show that, at the same strain level, the stress–amplitude response and the stress–strain hysteresis loops obtained from the small specimens are in excellent agreement with those from the standard cylindrical specimens. The fatigue curve of 6061Al derived from the small specimens also closely matches that obtained from the standard specimens. Despite a certain degree of scatter in the fatigue life data, the fatigue lives obtained from both types of specimens fall within a factor of 1.5 error band. The results indicate that, under the specific specimen configuration and material conditions investigated in this study, the proposed method can accurately characterize the low-cycle fatigue behavior of the material and yield fatigue life results consistent with those of the standard specimens.</p>