  Most studies for postselected weak measurement focus on using pure Gaussian
state as a pointer, which can only give an amplification limit reaching the
level of the ground state fluctuation. When the pointer is initialised in a
thermal state, we find that the amplification limit after the postselection can
reach the level of thermal fluctuation, indicating that the amplification
effect achieving the level of thermal fluctuation is also increased with the
temperature grow, and also give the amplification mechanism different from the
one with pure Gaussian state pointer. To illustrate these results, we propose
two schemes to implement room temperature amplification of the mechanical
oscillator's displacement caused by a single photon in optomechanical system.
The two schemes can both enhance the mechanical oscillator's original
displacement by nearly seven orders of magnitude, attaining sensitivity to
displacements of $ 0.26$ nm. Such amplification effect can be used to observe
the impact of a single photon on a room temperature mechanical oscillator which
is hard to detect in traditional measurement.
