The elasticity of a single polymer chain has been widely investigated in last decades. However, the direct measurement of the single polymer elasticity in an unperturbed state (i.e. inherent elasticity) remains a challenge. The main obstacle in this regard is that most force measurements are carried out in a liquid environment. The single polymer elasticity may be strongly affected by the complex interactions between solvent molecules and polymer such as van der Waals forces, hydrogen bonds and/or thermal motions. For instance, the single-chain elasticity of poly(ethylene glycol) (PEG) in water is different from that in nonpolar organic solvents, since hydrogen bonds can be formed between PEG and water molecules. In this study, the single-chain elasticity of PEG is investigated in high vacuum by means of atomic force microscopy (AFM)-based single-molecule force spectroscopy (SMFS). Solvent molecules and surface adsorbed water are removed thoroughly under high vacuum so that the situation is greatly simplified. PEG is dissolved in DI water to a concentration of 50 μg/mL, which is used for the polymer physisorption on a quartz substrate. Then, the sample is rinsed with abundant DI water to remove the loosely adsorbed polymer and dried by air flow. After that, the AFM chamber is pumped down to ~7.0×10-4 Pa to achieve high vacuum, where almost all adsorbed water molecules can be removed from the environment. The results show that PEG maintains its inherent elasticity in high vacuum, which can be well described by an elastic model of a single polymer chain (QM-FRC model) when F > 100 pN. In a nonpolar organic solvent (nonane), since there are only van der Waals (vdW) forces between solvent molecules and PEG, PEG presents an elasticity virtually identical to that in high vacuum. However, a slight difference can be observed in the low force region (F < 100 pN) in different environments. The long plateau (~45 pN) observed in high vacuum can be attributed to the adsorption/desorption force (mainly vdW force) of PEG on the substrate. It is greatly anticipated that the method used in the current study can be applied to investigate the inherent elasticity of other polymers in the future.