Abstract: Are there visible differences on safety level for structures designed according to Chinese existing design codes in different seismic fortification intensity regions? Margin ratio for ultimate limit capacity (MRULC) of structures, which is a parameter reflecting structural safety, is introduced, and then the structural safety is indicated by the probability density functions (PDFs) for MRULC. Following the idea of probability density evolution method, the transiting solution of PDF for MRULC is derived from two different descriptions of reliability, together with its approximation via a family of sequences. At last, taking four 9-story 3-bay reinforced concrete frames, which are designed for different seismic fortification intensity regions by codes, as examples, the PDFs of their MRULC are obtained by combining full-range nonlinear analysis with approximation via a family of sequences. By comparison, it is found that the shape of PDFs for MRULC is identical, whereas there are distinct differences for mean values, which manifest the discrepancies on safety level. What’s more, the structural safety level decreases as seismic fortification intensity increases. And this phenomenon should be paid enough attention for engineers.