  Critical fluctuations play a fundamental role in determining the spin orders
for low-dimensional quantum materials, especially for recently discovered
two-dimensional (2D) magnets. Here we employ the quantum decoherence imaging
technique utilizing nitrogen-vacancy centers in diamond to explore the critical
magnetic fluctuations and the associated temporal spin noise in van der Waals
magnet $\rm{Fe_{3}GeTe_{2}}$. We show that the critical fluctuation contributes
to a random magnetic field characterized by the noise spectra, which can be
changed dramatically near the critical temperature $T_c$. A theoretical model
to describe this phenomenon is developed, showing that the spectral density is
characterized by a $1/f$ noise near the $T_c$, while away from this point it
behaves like a white noise. The crossover at a certain temperature between
these two situations is determined by changing of the distance between the
sample and the diamond. This work provides a new way to study critical
fluctuation and to extract some of the critical exponents, which may greatly
deepen our understanding of criticality in a wide range of physical systems.
