The space-charge field with a higer-order nonlinearity in biased centrosymmetric photorefractive media is presented. When the higer-order term can be neglected,this space-charge field is that studied previously in biased centrosymmetric photorefractive media. The nonlinear wave equation with a higerorder nonlinearity is investigated in biased centrosymmetric photorefractive media,which can exhibit bright and dark spatial solitons under appropriate conditions. The nonlinear change in the refractive index in centrosymmetric photorefractive media resulted from the quadratic electro-optic effect,rather than the linear electro-optic effect in noncentrosymmetric media. The stability of these spatial solitons is discussed using beam propagation methods,and it has been found that optical spatial solitons in biased centrosymmetric photorefractive media are stable against small perturbations in amplitude not broken up.