  The origin of the knee in cosmic ray spectrum remains to be an unsolved
fundamental problem. There are various kinds of models which predict different
break positions and the compositions of the knee. In this work, we suggest to
use diffuse $\gamma$-rays and neutrinos as probes to test these models. Based
on several typical types of the composition models, the diffuse $\gamma$-ray
and neutrino spectra are calculated, which show distinctive cutoff behaviours
at energies from tens of TeV to multi-PeV. The expected flux will be observable
by the newly upgraded Tibet-AS$\gamma$+MD (muon detector) experiment as well as
more sensitive future projects, such as LHAASO and HiSCORE. By comparing the
neutrino spectrum with the recent observations by IceCube experiment, we find
that the diffuse neutrinos from interactions between the cosmic rays and the
interstellar medium may not be responsible to the majority of the IceCube
events. Future measurements of the neutrinos may be able to identify the
Galactic diffuse component and further shed light on the problem of the knee of
cosmic rays.
