In the light scattering method, the small dynamic range of the scattering signal will directly affect the measurement accuracy of optical element surface roughness. To solve the problem, the image fusion is applied to acquire large dynamic range scattering signal. First, the scattering images of different exposure times at the same position are collected. Then, it is transformed with wavelet transform, the decomposed low-frequency components are fused using the principle of strongest scattering signal, meanwhile high-frequency components are fused using the principle of regional characteristic measurement. The multi-exposure scattering image fusion is completed by inverse wavelet transform. Finally, the large dynamic range scattering signal is processed to obtain the surface roughness value of the component. The results show that the dynamic range of the fused scattering signal is increased by 3 orders of magnitude compared with the unfused scattering signal. The surface roughness of optical elements calculated from the fused scattering signal is consistent with the measurement result of the white light interferometry, which proves the proposed fusion method can be used for large dynamic range scatter signal acquisition and surface roughness measurement of optical components.