Abstract: A practical method for analyzing the reliability of existing structural systems is put forward. The minimum value of the storey reliability is suggested to be the structural reliability. The storey reliability is constifuled of the one of each member by weight function, which is combined by the ratio of load vs. resistance and other modified coefficent. The aseismic benefit function of existing reinforced concrete frame structure is established based on the shear-resisting capacity of columns according to the previous earthquake damage. T he influence factors of carbonization of concrete and structural load are analyzed. A practical case of reinforced concrete structure is given herein, the aseismic reliability of the orig inal structure and those of the structure strengthened by three different methods are calculated respectively.