  In order to evaluate the ground-based infrared telescope sensitivity affected
by the noise from the atmosphere, instruments and detectors, we construct a
sensitivity model that can calculate limiting magnitudes and signal-to-noise
ratio ($S/N$). The model is tested with tentative measurements of $\rm M'$-band
sky brightness and atmospheric extinction obtained at the Ali and Daocheng
sites. We find that the noise caused by an excellent scientific detector and
instruments at $-135^\circ \rm C$ can be ignored compared to the $\rm M'$-band
sky background noise. Thus, when $S/N=3$ and total exposure time is 1 second
for 10 m telescopes, the magnitude limited by the atmosphere is $13.01^{\rm m}$
at Ali and $12.96^{\rm m}$ at Daocheng. Even under less-than-ideal
circumstances, i.e., the readout noise of a deep cryogenic detector is less
than $200e^-$ and the instruments are cooled to below $-87.2^\circ \rm C$, the
above magnitudes decrease by $0.056^{\rm m}$ at most. Therefore, according to
observational requirements with a large telescope in a given infrared band,
astronomers can use this sensitivity model as a tool for guiding site surveys,
detector selection and instrumental thermal-control.
