Although BiFeO3-based photovoltaic devices have currently attracted much attention due to their unique physical properties, their practical applications have been limited by the complex and obscure intrinsic physical mechanisms. This paper reported the synthesis of the Ni, Sm co-doped BiFeO3 film capacitors by a spin-coating technology. The results and analysis showed that co-doped BiFeO3 films exhibited low leakage current and well saturated ferroelectric hysteresis loops. Especially, the photovoltaic effect and diode-like effect of ferroelectric film capacitors could be efficiently modulated by the external polarization. The complex intrinsic physical mechanism of photovoltaic effect and diode-like effect under polarization modulation was studied and explained clearly by the energy-band diagram and the theory of ferroelectric polarization. Moreover, the effect of inherent and external factors on photovoltaic output of BiFeO3 films was jointly analyzed by the ferroelectric polarization and photovoltaic mechanism. This theoretical exploration may facilitate to improve the understanding of the photovoltaic effect in ferroelectrics, which will be likely to have the opportunity to advance the design of switchable devices that combine ferroelectric and photovoltaics.