The coherent longitudinal acoustic (LA) phonons in La and Nb codoped polycrystalline BiFeO3 (Bi0.8La0.2Fe 0.99Nb0.01O3 (BLFNO)) films are photo-induced and detected by the ultrafast reflectance spectroscopy. The generation mechanism of LA phonons is strongly connected with the ferroelectric polarization and is attributed to the transient photostriction effect, which is a combination of the optical rectification effect and the electrostriction effect. The strain modulation of sound velocity and out-of-plane elastic properties are demonstrated in BLFNO film on SrTiO3, which gives the insight on the dynamical coupling between electrical polarization and lattice deformation. Our findings are desired for the design of BiFeO3-based photo-driven remote control micro/nano devices. 2012 American Institute of Physics.