<p>Earthquake causes severe impact in the social property, and its damage to the aged buildings is one of the most disastrous consequences. Thus, seismic upgrading techniques and advanced design approaches are continuously proposed for a better structural performance, of which the externally-attached sub-system is an effective strategy. The authors formerly proposed a high-performance external sub-system for seismic upgrading, namely, the self-centering precast bolt-connected steel-plate reinforced concrete (SC-PBSPC) buckling-restrained braced frame (BRBF), and experiments have been performed to validate its feasibility and superiority. In this paper, a novel design framework for the external SC-PBSPC BRBF sub-system considering combined demand-capacity uncertainties under nonstationary excitation is further investigated. The novel design framework is derived from the incremental dynamic analysis and capacity spectrum method (IDA-CSM), and it is an organic integration and an effective improvement for a better upgrading solution. The proposed stochastic IDA-CSM based design framework consists of three stages (i.e., the performance evaluation before upgrading, the component design of sub-systems, and the performance verification after upgrading) and two primary analyzing approaches [i.e., the stochastic pushover analysis (POA) for capacity-demand spectra curves, and the stochastic IDA for fragility curves]. During the procedure, the design factors are regarded to be stochastic variables and multiple uncertainties are incorporated for a probabilistic estimation, both before and after upgrading, respectively. An application example via a three-dimensional frame building is also implemented to verify the feasibility of the design framework, and multiple uncertainties provide a probabilistic prediction of the upgraded behavior in a more realistic way. The proposed research serves as a reference for the related design exploration and plays a key role for further probabilistic work from a macro perspective.</p>