The application based on composite scattering of the actual sea surface with the target below of it demand in ocean engineering, radiation calibration and underwater target detection etc. The application based on composite scattering of the actual sea surface with the target below of it demand in ocean engineering, radiation calibration and underwater target detection etc. The improved onedimensional fractal model is adopted to simulate the actual sea surface, the method of moments is used to study the composite electromagnetic scattering about the sea surface and the trapezoidal crosssection cylinder target beneath it, the curve is abtained by numerical calculation about the complex scattering coefficient changes with the scattering angle, the impact on the composite scattering coefficient is discussed in detail about the surface wind speed, the surface fractal dimension, water temperature, the size and depth of the target and the frequency of the incident wave, the result show that the influence of the surface wind speed, the fractal dimension and the frequency of the incident wave are obvious to the composite scattering coefficient, that the water temperature, the size and depth of the target coefficient have little effect on it.