To study the modulation instability of one-dimensional broad optical beams in biased two-photon photovoltaic photorefractive crystals under steady-state conditions,we obtain the one-dimensional modulation instability growth rates by globally treating the space-charge field.The characteristics of the one-dimensional modulation instability growth rates have been numerically investigated.The numerical results show that the modulation instability growth rates will increase as the external electric field enhancing for positive electric field whereas the growth rates will decrease as the absolute value of the external electric field enhancing for negative electric field when the incident optical intensity is a constant.When the external electric field is given,the modulation instability growth rates will increase as the incident optical intensity enhancing.When the external field is absent,the modulation instability in two-photon photovoltaic photorefractive crystals can be deduced.When the photovoltaic effect is neglectable,we obtain the modulation instability growth rates in biased non-photovoltaic photorefractive crystals.