In order to investigate the optimal shear span ratio of reinforced concrete (RC) columns applied to flexible isolation layers, a comparative analysis based on numerical simulation has been made of the ductility coefficient, the energy dissipation coefficient, and the lateral stiffness between monolithic columns and split columns. The results show that the ratios of the ductility coefficient between monolithic columns and split columns increase then decrease with the increase of its shear span ratios, with the maximum value of the latter achieved in the range of 1.5 to 2.0; when the shear span ratio is less than one point five, the ratios of the energy dissipation coefficient between monolithic columns and split columns increase slowly but not dramatically with the increase of its shear span ratios; when the shear span ratio is greater than or equal to one point five, the ratios of the energy dissipation coefficient between monolithic columns and split columns increase rapidly; the ratios of the lateral stiffness between monolithic columns and split columns increase with its shear span ratios, which are inversely proportional to the isolation effect. Thus, a final conclusion has been drawn: the optimal shear span ratio should be in the range of 1.5~2.0. Keywords ：split columns；shear span ratio；ductility coefficient；energy dissipation coefficient；lateral stiffness