We present an image fusion method for determining optical length in radiography of dense objects. We first show that the scatter received in the screen-films (SFs) can be treated as a constant when a grade collimation is arranged properly in the radiographic image system. Then the nonlinear radiographic imaging process can be demonstrated by a monotonous nonlinear function, which is determined by the whole system. The optical length of the fused image is derived from a weighted sum of the optical length conversed from the single image directly, and the weight of the optical length is determined by the greatest derivation of the function. Experiment results show that the algorithm has improved the signal-to-noise ratio (SNR) of the fused image and extended the dynamic range of the receivers.