  We show that topological nodal points can emerge in photonic crystal
possessing mirror symmetry. The mechanism of generating topological nodal
points is discussed in a two-dimensional photonic square lattice, in which four
topological nodal points split out naturally after the touching of two bands
with different parity. The emergence of such nodal points, characterized by
vortex structure in momentum space, is attributed to the unavoidable band
crossing protected by mirror symmetry. The topological nodes can be unbuckled
through breaking the mirror symmetry and a photonic Chern insulator can be
achieved through time reversal symmetry breaking. The joint effect of breaking
time reversal symmetry and breaking inversion symmetry is further found to
strengthen the finite size effect, providing ways to engineer helical edge
states.
