  Mass sensing connects the mass variation to a frequency shift of a mechanical
oscillator, whose limitation is determined by its mechanical frequency
resolution. Here we propose a method to enlarge a minute mechanical frequency
shift, which is smaller than the linewidth of the mechanical oscillator, into a
huge frequency shift of the normal mode. Explicitly, a frequency shift of about
20 Hz of the mechanical oscillator would be magnified to be a 1 MHz frequency
shift in the normal mode, which increases it by five orders of magnitude. This
enhancement relies on the sensitivity appearing near the quantum critical point
of the electromechanical system. We show that a mechanical frequency shift of 1
Hz could be resolved with a mechanical resonance frequency $\omega_b = 11\times
2\pi $ MHz. Namely, an ultrasensitive mechanical mass sensor of the resolution
$\Delta m /m \sim2\Delta \omega_b/\omega_b\sim 10^{-8}$ could be achieved. Our
method has potential application in mass sensing and other techniques based on
the frequency shift of a mechanical oscillator.
