  Using the non-equilibrium Green function method, we study the Andreev
reflection in a Y-shaped graphene-superconductor device by tight-binding model.
Considering both the zigzag and armchair terminals, we confirm that the zigzag
terminals are the better choice for detecting the Andreev reflection without no
external field. Due to scattering from the boundaries of the finite-size centre
region, the difference between Andreev retroreflection and specular reflection
is hard to be distinguished. Although adjusting the size of the device makes
the difference visible, to distinguish them quantitatively is still impossible
through the transport conductance. The problem is circumvented when applying a
perpendicular magnetic field on the centre region, which makes the incident
electrons and the reflected holes propagate along the edge or the interface. In
this case, the retroreflected and specular reflected holes from the different
bands have opposite effective masses, therefore the moving direction of one is
opposite to the other. Which external terminal the reflected holes flow into
depends entirely on the kind of the Andreev reflection. Therefore, the specular
Andreev reflection can be clearly distinguished from the retroreflected one in
the presence of strong magnetic field, even for the device with finite size.
