In order to satisfy impact angle constraints and reduce energy cost under salvo attacks against high-speed non-maneuvering targets， a three-dimensional coupled model is established， and a 3D energy optimal guidance law for non-linear systems with large constrained impact angles is proposed. The reasons for the low accuracy of 3D decoupled model are discussed， and a 3D coupled model is established. Based on this model， the appropriate control variables are chosen， and the 3D energy optimal guidance law under large impact angle constraints is derived， based on minimum principle. Experimental results indicate that the performance of the proposed coupled model on miss distance， impact angle error and energy cost has improved 0.7 m，1.1°，200 g compared with decoupled model. It can prove the advantages of the guidance law on control precision and energy cost.