In this paper, we proposed to observe a phonon blockade in multimode optomechanical system. The multimode optomechanical system is consisting of one mechanical mode driven by a weakly mechanical field and two optical modes driven by two optical fields (a weak one and a strong one). Under the interaction of the strong optical driving field, the multimode optomechanical system can be reduced to a much simple model for a mechanical mode linearly coupled to an optical mode with Kerr nonlinearity. Our calculations show that strong phonon antibunching effects can be observed even with weak optomechanical coupling. This counter-intuitive phenomenon, i.e., unconventional phonon blockade, results from the destructive interference between different paths for two-phonon excitation and the optimal conditions for unconventional phonon blockade are obtained analytically. Moreover, the statistical properties of the phonons can be controlled by regulating the strength ratio and the relative phase between the weakly driving fields, and this provides us an effective way to realize tunable single-phonon sources. Finally, we show that the thermal phonons have a detrimental impact on the unconventional phonon blockade and a proper increase of the strengths of the weakly driving fields can be helpful to overcome the detrimental impact induced by the thermal phonons.