Bi1-xSmxFeO3 thin films (x=0, 0.05, 0.10, 0.15) on Pt(111)/Ti/SiO2/Si substrates were fabricated by the sol-gel process and the effect of Sm doping on their crystal structure was studied by synchrotron radiation X-ray diffraction and Raman spectroscopy. It is revealed that a phase transition from rhombohedral to orthorhombic structure takes place with increasing Sm content, resulting in two-phase coexistence at x=0.10. The phase transition can be ascribed to the difference between the smaller radius of Sm3+ than its substituted Bi3+. Meanwhile, the composition-dependent dielectric, ferroelectric and piezoelectric properties were also investigated. PFM scanning and switching spectroscopy results confirmed the enhancement of piezoresponse at x=0.10 corresponding to the R-O morphotropic phase boundary (MPB) region. The ferroelectric properties of Sm-doped BiFeO3 films were found to decrease with increasing Sm content, indicating that the extrinsic piezoelectric response contributes more to the improved piezoelectricity at MPB.