Theoretical and experimental studies on the coupling at laser diode facet facinglong external cavity have been carried out. The analytical expressions of the couplingcoefficient has been derived. It can be divided into two factors, one is due tO the Off overlapping between the centers of the diode mode and its reflected counterpart, and theother is due to the alteration of the wave vector after being dispersed by the grating. Boththeoretical and experimental verifications show that is a Gaussian function.