  $\mathcal{PT}$-symmetry-breaking enhanced cavity optomechanical magnetometer
is proposed, which is achieved by monitoring the change of intensity of a
nonlinear four-wave mixing (FWM) process in a gain-cavity-assisted cavity
optomechanical system (COMS). Compared with the traditional single loss COMS,
the FWM intensity can be enhanced by two orders of magnitude when the
gain-cavity-assisted COMS operates at the $\mathcal{PT}$-symmetry-breaking
phase. Meanwhile, the sensitivity of magnetic field sensing can be increased
from $10^{-9}$T to $10^{-11}$T. This originally comes from the fact that the
effective detuning and decay of loss-cavity can be effectively modified in the
$\mathcal{PT}$-symmetry-breaking phase. Our work shows that an
ultrahigh-sensitivity magnetometer can be achieved in the
$\mathcal{PT}$-symmetry-breaking COMS, which will have wide applications in the
field of quantum sensing.
