A new fast far field approximation algorithm is introduced to calculate the composite scattering characteristics of rough surface and the target in near field, by using the reduced-order electromagnetic current iteration method. Also we use the multi-process parallel algorithm to reduce the simulation time and analyze the influence on the results of the reduced-order electromagnetic current iteration. The calculated results by the algorithm described in the paper and the results obtained by multi-level fast multi-pole method (MLFMM) are in good agreement, so the effectiveness of the algorithm is verified. At the same time, the use of tank to carry out the measurements of near-field complex scattering between the targets and rough surfaceshows that the test results and the calculation results are very consistent.