  Fermi arc surface states are the hallmark of Weyl semimetals, whose
identification is usually challenged by their coexistence with gapless bulk
states. Surface transport measurements by fabricating setups on the sample
boundary provide a natural solution to this problem. Here, we study the Andreev
reflection (AR) in a planar normal metal-superconductor junction on the Weyl
semimetal surface with a pair of Fermi arcs. For a conserved transverse
momentum, the occurrence of normal reflection depends on the relative
orientation between the Fermi arcs and the normal of the junction, which is a
direct result of the disconnected Fermi arcs. Consequently, a crossover from
the suppressed to perfect AR occurs with varying the orientation of the planar
junction, giving rise to a change from double-peak to plateau structure in
conductance spectra. Moreover, such a crossover can be facilitated by imposing
a magnetic field, making electrons slide along the Fermi arcs so as to switch
between two regimes of the AR. Our results provide a decisive signature for the
detection of Fermi arcs and open the possibilities of exploring novel
phenomenology through their interplay with superconductivity.
