Aerolysin is a bacterial toxin that forms transmembrane pores at the host plasma membrane and has a narrow internal diameter and great stability. These assets make it a highly promising nanopore for detecting biopolymers such as nucleic acids and peptides. Although much is known about aerolysin from a microbiological and structural perspective, its membrane association and pore-formation mechanism are not yet fully understood. Here, we used angle-resolved second harmonic scattering (AR-SHS) and single-channel current measurements to investigate how wild-type (wt) aerolysin and its mutants interact with liposomes in aqueous solutions at femtomolar concentrations. Our AR-SHS experiments were sensitive enough to detect changes in the electrostatic properties of membrane-bound aerolysin, which were induced by variations in pH levels. We reported for the first time the membrane binding affinity of aerolysin at different stages of the pore formation mechanism: while wt aerolysin has a binding affinity as high as 20 fM, the quasi-pore and the prepore states show gradually decreasing membrane affinities, incomplete insertion, and a pore opening signature. Moreover, we quantitatively characterized the membrane affinity of mutants relevant for applications to nanopore sensing. Our study provides a label-free method for efficiently screening biological pores suitable for conducting molecular sensing and sequencing measurements as well as for probing pore-forming processes.