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Phase transition from the over-damping to under-damping states is a ubiquitous phenomenon in physical systems. However, what kind of symmetry is broken associated with this phase transition remains unclear. Here, we discover that this phase transition is determined by an anti-parity-time (anti-$\mathcal{PT}$) symmetry hidden in a single damping linear resonator, which is significantly different from the conventional anti-$\mathcal{PT}$-symmetric systems with two or more modes. We show that the breaking of the anti-$\mathcal{PT}$ symmetry yields the phase transition from the over-damping to under-damping states, with an exceptional point (EP) corresponding to the critical-damping state. Moreover, we propose an optomechanical scheme to show this anti-$\mathcal{PT}$ symmetry breaking by using the optical spring effect in a quadratic optomechanical system. We also suggest an optomechanical sensor with the sensitivity enhanced significantly around the EPs for the anti-$\mathcal{PT}$ symmetry breaking. Our work unveils the anti-$\mathcal{PT}$ symmetry hidden in damping oscillations and hence opens up new possibilities for exploiting wide anti-$\mathcal{PT}$ symmetry applications in single damping linear resonators.