Aiming at the coupling of bank-to-turn (BTT) missile, a non-decoupling three-dimensional (3D) guidance law was proposed. Firstly, the 3D guidance model was established after the line of sight (LOS) angles velocity model was established through vectors description, and the azimuth' change of LOS was described based on the twist method. Secondly, the guidance model was simplified and the linear model was derived on the assumption that LOS angles' change is slow. Finally, the 3D optimal guidance laws were designed respectively using the quadratic optimal control, corresponding to no ending constraints and ending constraints. The proposed guidance law can be satisfied with the guidance precision of BTT missile since it is able to avoid the information loss caused by the decoupling method.