Although pi-pi interactions have been studied for several decades, the quantification of the strength of pi-pi interactions in a macromolecule remains a big challenge. Herein, we utilize single-molecule atomic force microscopy and steered molecular dynamics simulations to study the pi-pi interactions in polystyrene (PS). It is found that in high vacuum, the single-chain mechanics of PS differs largely from that of polyethylene (PE). Accordingly, the strength of intrachain pi-pi interactions in PS is estimated to be 0.7 kcal/(mol stack), which is much lower than that in a small-molecule system (benzene dimer, 2-3 kcal/(mol stack)). Further study shows that in high vacuum, there are two types of pi-pi stacking in the single PS chain, i.e., the every-othermoiety (E) type and the adjacent-moiety (A) type. Upon force stretching, a transition from E-type to A-type pi-pi stacking can be observed.