The majority of limestone reservoirs in China are characterized by deep burial，low porosity，and low permeability and are influenced by factors such as elevated temperatures，high closure pressures，and strong heterogeneity. Conventional hydrochloric acid fracturing techniques often fail to achieve non-uniform etching on such reservoirs， resulting in poor transformation effects and hindering the efficient development of such reservoirs. The acid fracturing technology of multi-stage alternating injection was used to facilitate non-uniform etching of fracture wall surfaces by iteratively forming viscous fingering in the prepad. However，the law governing the acid-etching fracturing of multi-stage alternating injection remains unclear，and the acid injection stage boundaries are not determined. At present， acid fracturing design typically relies on empirical approaches. To address these gaps，this study built an acid fracturing model of multi-stage alternating injection for limestone reservoirs by considering the influence of viscosity variation and alternating stages based on a two-scale continuous acidification model. Based on the acid fracturing model of multistage alternating injection，the influence of acid viscosity，acid injection rate，and acid injection stages on the acid fracture effects was analyzed by combining the acid rock reaction kinetics，acid-etched fracture experimental data，and numerical simulation results.The acid-etching law of multi-stage alternating injection in limestone reservoirs and the acid injection stage boundaries were determined， and the breakthrough volume of acid fluid was introduced to provide the basis for site optimization. The findings suggest that to enhance the non-uniform etching of fracture wall surfaces and obtain high conductivity of acid-etching fractures，an alternating injection process involving hydrochloric acid and thickened acid should be adopted for pure limestone reservoirs at a formation temperature of 90 °C. 0.4% HPAM-thickened acid should be added when the acid injection rate is 5 m³/min，with an optimal acid injection stage of no more than three. At this time，the breakthrough volume of acid fluid is the smallest，which is suitable for on-site construction.