We experimentally study the ac Stark splitting in D2 line of cold 87 Rb atoms. The frequency span between the Autler-Townes doublets is obviously larger than that derived from theoretical calculation. Two physical effects,which increase the effective Rabi frequency, contribute to the splitting broadening. First, atoms tend to distribute in strong field places of a inhomogeneous red-detuned light field. Second, atoms reabsorb scattered light when they are huge in number and high in density.