A terminal structure with optimized reliability for a power semiconductor device, a preparation method therefor, application thereof, a power device and a preparation method therefor are provided. The terminal structure includes a phosphorus-doped silicon oxide layer, a silicon nitride layer, a silicon-rich silicon nitride semi-insulating layer, an undoped silicon dioxide layer, and an organic medium layer. The silicon-rich silicon nitride semi-insulating layer is of an alternating superposition structure of a silicon-rich silicon nitride layer and an ultra-thin silicon nitride barrier layer. The terminal structure effectively prevents external moisture from invading the power device, which improves the robustness of the power device under a moisture condition. The multi-layer silicon-rich silicon nitride is used as a semi-insulating layer, which makes an electric field on a surface of the power device evenly distributed in gradient, prevents the electric field from being gathered at a device terminal.