  A long-standing debate on Unruh effect is about its obscure thermal nature.
In this Letter, we use quantum Fisher information (QFI) as an effective probe
to explore the thermal nature of Unruh effect from both local and global
perspectives. By resolving the full dynamics of UDW detector, we find that the
QFI is a time-evolving function of detector's energy gap, Unruh temperature
$T_U$ and particularities of background field, e.g., mass and spacetime
dimensionality. We show that the asymptotic QFI whence detector arrives its
equilibrium is solely determined by $T_U$, demonstrating the global side of
Unruh thermality alluded by the KMS condition. We also show that the local side
of Unruh effect, i.e., the different ways for the detector to approach the same
thermal equilibrium, is encoded in the corresponding time-evolution of the QFI.
In particular, we find that with massless scalar background the QFI has unique
monotonicity in $n=3$ dimensional spacetime, and becomes non-monotonous for
$n\neq3$ models where a local peak value exists at early time and for finite
acceleration, indicating an enhanced precision of estimation on Unruh
temperature at a relative low acceleration can be achieved. Once the field
acquiring mass, the related QFI becomes significantly robust against the Unruh
decoherence in the sense that its local peak sustains for a very long time.
While coupling to a more massive background, the persistence can even be
strengthened and the QFI possesses a larger maximal value. Such robustness of
QFI can surely facilitate any practical quantum estimation task.
