In this paper, we carried out an improved kinematic low energy electron diffraction and constant momentum transfer averaging (KLEED/CMTA) analysis of aSi(111)31/2×31/2-Al structure After a thorough optimization of the parameters, we came to the conclusion that T4 model is the best, which gives an excellent agreement with experiment curves(RVHT = 0.158). Atomic coordinates and bond lengths of the model have been determined for Al atoms and all Si atoms in the first six layers of the substrate. None of the bond-length deviations from the bulk value is larger than 5% and within an error bar of 0.05A, all of the parameters (except a lateral displacement) are consistent with the conclusion of DLEED. The successful application of the KLEED/CMTA method to such a complicated surface structure indicates strongly that the method is really very poweriul and reliable,and thus has great potential in surface structure analysis.