  We consider a matter wave packet of cold atom gas impinging upon a step
potential created by the optical light field. In the presence of spin-orbit
(SO) coupling, the atomic eigenstates contain two types of evanescent states,
one of which is the ordinary evanescent state with pure imaginary wave vector
while the other possesses complex wave vector and is recognized as oscillating
evanescent state. We show that the presence and interplay of these two types of
evanescent states can give rise to two different mechanisms for total internal
reflection (TIR), and thus lead to unusual Goos-H\"anchen (GH) effect. As a
result, not only large positive but also large negative GH shift can be
observed in the reflected atomic beam. The dependence of the GH shift on the
incident angle, energy and height of the step potential is studied numerically.
