A semiconductor epitaxial structure is provided, which includes: a nitride nucleation layer, formed on a substrate including silicon, sapphire, patterned sapphire substrate (PSS) or silicon carbide, a nitride layer on the nitride nucleation layer and an multi-layer structure in the nitride layer. The multi-layer structure includes a first intermediate layer and a second intermediate layer formed on the first intermediate layer. The first intermediate layer includes AlGaN, the second intermediate layer includes AlGaN or aluminum nitride, and the average composition of Al in the first intermediate layer is less than that in the second intermediate layer. A method for forming a semiconductor epitaxial structure is provided. The semiconductor epitaxial structure according to the present disclosure can not decrease the crystalline quality when a compressive stress is introduced, which may avoid a crack phenomenon or quality degradation caused by the change of temperature.