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Before 1980s, the circular suspension spring in automobile subjected to torsion fatigue load, under the cyclic normal tensile stresses, the majority of fatigue fracture occurred was in normal tensile fracture mode(NTFM) and the fracture surface was under 45°diagonal. Because there exists the interaction between the residual stresses induced by shot peening and the applied cyclic normal tensile stresses in NTFM, which represents as “stress strengthening mechanism”, shot peening technology could be used for improving the fatigue fracture resistance(FFR) of springs. However, since 1990s up to date, in addition to regular NTFM, the fatigue fractures occurred of peened springs from time to time are in longitudinal shear fracture mode(LSFM) or transverse shear fracture mode(TSFM) with the increase of applied cyclic shear stresses, which leads to a remarkable decrease of FFR. However, LSFM/TSFM can be avoided effectively by means of shot peening treatment again on the peened springs. The phenomena have been rarely happened before. At present there are few literatures concing this problem. Based upon the results of force analysis of a spring, there is no interaction between the residual stresses by shot peening and the applied cyclic shear stresses in shear fracture. This means that the effect of“stress strengthening mechanism” for improving the FFR of LSFM/TSFM is disappeared basically. During shot peening, however, both of residual stress and cyclic plastic deformed microstructure are induced synchronously like“twins”in the surface layer of a spring. It has been found for the first time by means of force analysis and experimental results that the modified microstructure in the“twins”as a“structure strengthening mechanism” can improve the FFR of LSFM/TSFM. At the same time, it is also shown that the optimum technology of shot peening strengthening must have both“stress strengthening mechanism”and“structure strengthening mechanism”simultaneously so that the FFR of both NTFM and LSFM/TSFM can be improved by shot peening.