Formation of structure of Zn-5wt%Al alloy during the nonequilibrium solidification was investigated. Results show that the primary Al-rich phase(α phase) is precipitated during the nonequilibrium solidification. With increasing of the cooling rate, the primary Al-rich phase(α phase) not only becomes less and less but also transforms from cellular crystal to dendritic crystal gradually. Based on this, the eutectic symbiotic region of Zn-5wt%Al alloy phase diagram was researched, and the qualitative phase diagram of eutectic symbiotic region was lined out. In addition, results show that the hypoeutectic structure which is crystallized during the nonequilibrium solidification hasn’t evident core under high cooling rate, and the lamella of hypoeutectic structure is disordered.