The value of sensitive factors in structural reliability analysis greatly affects reliability calculation results. These factors must be accurately determined and studied to improve the accuracy of calculated results. Based on the analysis of existing sensitivity calculation methods, a new practical sensitive factor analysis method was put forward to analyzing sensitive factors in structural strength reliability calculations. The dimensionless sensitive indexes of structural reliability were defined in the new method to compare the influence degree of random parameters to reliability. And the method was applied to analyze sensitivity under the two conditions of the structural resistance decreasing and not changing with time. Random variables, defined by their probability distribution and corresponding distribution parameters, were used to describe uncertainties in the reliability calculation. Different forced state RC columns were taken as examples to analyze the structural sensitivity, and sensitive factors affecting the columns’ structural reliability were obtained. The results show that the reliability of time-varying structure has descended to some extent compared with that without time-varying structure, and the effect from the random parameters to reliability index. The example and theory demonstrate the feasibility and rationality of the new method. This article has certain reference value to the structural design, construction and detection.