  Based on the scattering matrix method, we theoretically demonstrate that the
chiral interaction can induce the almost perfect photon blockade (PB) in the
waveguide-cavity quantum electrodynamics (QED) system. The mechanism relies on
the multi-photon-paths interference within the waveguide, which is clearly
shown by the analytical parameter regime for $g^{(2)}(0)\approx0$. When $N$
cavities are introduced into the system, there are $N$ optimal parameter points
accordingly for the almost perfect PB, and the required chirality decreases
exponentially with increasing $N$. Under the conditions of resonant driving and
specific chirality, the output light only relies on the parity of $N$
($N\ge2$), where the coherent state and single-photon state correspond to the
case of system including the odd and even number of cavities, respectively. Our
work offers an alternative route for achieving almost perfect PB effects by
employing the chirality of system, with potential application in the on-chip
single-photon source with integrability.
