Identifying the solid electrolyte interphase (SEI) components in all-solid-state lithium batteries (ASSLBs) is essential when developing strategies for improving this battery technology. However, a comprehensive understanding of the interfacial stability and decomposition reactions of solid polymer electrolyte with lithium metal anode remains a challenge, not least outside the dominating poly(ethylene oxide)-based materials. Here, we report the reactivity of an electrolyte system composed of a polyester (poly-epsilon-caprolactone, PCL) and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) salt on Li (100) surface, and the subsequent SEI formation, using ab initio molecular dynamics (AIMD) simulations. The step-by-step electrolyte decomposition on the anode surface is monitored, and the resultant major SEI components are analyzed by Bader charges to correlate with X-ray photoelectron (XPS) signal. The presence of PCL at the Li surface promotes a rapid initial reduction of LiTFSI salt via cleavage of S-N and C-S bonds, and its complete dissociation and formation of major SEI components such as LiF, Li2O, Li2S, and C-containing species. Furthermore, a computational analysis of relevant XPS spectra is performed to support the degradation compounds.