We demonstrate a PT -symmetry-breaking chaos in optomechanical system (OMS), which features an ultralow driving threshold. In principle, this chaos will emerge once a driving laser is applied to the cavity mode and lasts for a period of time. The driving strength is inversely proportional to the starting time of chaos. This originally comes from the dynamical enhancement of nonlinearity by field localization in PT -symmetry-breaking phase (PT BP). Moreover, this chaos is switchable by tuning the system parameters so that a PT -symmetry phase transition occurs. This work may fundamentally broaden the regimes of cavity optomechanics and nonlinear optics. It offers the prospect of exploring ultralow-power-laser triggered chaos and its potential applications in secret communication.PACS numbers: 07.10.Cm Cavity optomechanics is a rapidly developing research field exploring the radiation-pressure interaction between the electromagnetic and mechanical systems [1]. Thanks to this nonlinear interaction, the optomechanical system (OMS) provides an alternative platform for implementing many interesting quantum [2][3][4][5][6][7][8] and classical [9-17] nonlinearity phenomena. In particular, one can strong driving the cavity so that the OMS enters into a regime of self-induced oscillations [9][10][11][12], where the backaction-induced mechanical gain overcomes mechanical loss. Further increasing the strength of the driving laser, chaotic motion emerges both in the optical and mechanical modes requiring no external feedback or modulation [13][14][15][16]. It is useful for generating random numbers [18] and implementing secret information processing [19][20][21]. However, to apply chaos into the secret communication scheme requiring low-power optical interconnects, the chaos threshold should be reduced dramatically [22,23].The notion of parity-time (PT ) symmetry, initially proposed in quantum mechanics [24], attracts recently enormous attention in the field of optics [25][26][27][28][29][30][31][32][33][34][35][36][37][38][39]. It is known [24] that PT symmetric Hamiltonians can exhibit a real eigenvalue spectrum in spite of the fact that they can be non-Hermitian. These systems have the interesting property to undergo an abrupt phase-transition where the system loses its PT-symmetry. At the exceptional point (EP) pairs of eigenvalues collide and become complex. Typically, the transition between PT symmetric phase (real spectrum) to spontaneously broken PT symmetry (complex spectrum) occurs as a parameter in the Hamiltonian (which somehow controls the degree of non-Hermiticity) is varied [25][26][27]. This phase transition has been demonstrated in synthetic waveguides and microcavities, and can induce unique optical phenomena including loss-induced transparency [28], power oscillations violating left-right symmetry [29], low-power optical diodes [35] and single-mode laser [36,37]. A natural question is whether it could influence the chaos dynamics (especially the chaos threshold) significantly. Moreover, the crossover b...