Shale reservoirs are characterized by low porosity，low permeability，and difficulty in exploitation，typically requiring the development approach combining long horizontal wells with large-scale volume fracturing techniques. After the fracturing fluid enters the formation，it undergoes a series of physical and chemical reactions with the reservoir，where salt ion diffusion plays a significant role. However，the existing research on the mechanism of ion diffusion and its application to diagnosing hydraulic fracture network is still imperfect. Therefore，this paper combines theoretical analysis with field cases to summarize the sources of salt ions in shale reservoirs，and the characteristics，mechanisms and influencing factors of ion diffusion，and presents an example of diagnosing hydraulic fracture network based on salt ion diffusion. The sources of salt ions in shale reservoirs include dissolved salts in the water film on the pore walls，crystallized salts from hydrocarbon generation and water expulsion，and salts produced by water-rock interactions. The characteristics of ion diffusion are similar to the imbibition process，which is divided into three stages：initial，transitional，and late diffusion. The initial ion diffusion rate is relatively high，exhibits a linear relationship on a logarithmic time scale，and follows Fick's law and the Einstein-Smoluchowski equation. The factors affecting salt ion diffusion include reservoir properties，solution properties，and other factors such as temperature and viscosity. Saltion diffusion can be used for the diagnosis of the development degree of volume fracturing fracture network. The method of diagnosing hydraulic fracture network through salt ion diffusion can provide scientific guidance for shale reservoir fracturing design and fracture network evaluation，help to deliver more accurate and effective reservoir stimulation and promote the efficient development and utilization of shale oil and gas resources.