Alzheimer's disease (AD) is the most common neurodegenerative disease among the elderly. Abnormal aggregates of both β-amyloid peptide (Aβ) subtypes, Aβ and Aβ, are the typical neuropathology hallmarks of AD. However, because of the lack of specific recognition elements such as an antibody and aptamer, it is difficult to differentiate and determine the oligomers of Aβ and Aβ in clinic. In this paper, we developed a planar bilayer lipid membrane (BLM)-based electrochemical biosensor. According to the dynamic differences on oligomer-induced BLM damage, both low-mass, soluble oligomers of Aβ and Aβ (L-AβO and L-AβO) were measured in turn by electrochemical impedance spectroscopy. The BLM was supported by a porous 11-mercaptoundecanoic acid layer on a gold electrode, which amplified the impedance signal corresponding to the membrane damage and improved the detection sensitivity. The weakly charged surface of the BLM ensured the low non-specific adsorption of coexisting proteins in cerebrospinal fluid (CSF). Using the electrochemical biosensor, L-AβO was determined within 20 min, with a linear range from 5 to 500 pM and a detection limit of 3 pM. Meanwhile, L-AβO was determined within 60 min, with a linear range from 60 pM to 6.0 nM and a detection limit of 26 pM. The recoveries in oligomer-spiked artificial CSF and human CSF samples confirmed the accuracy and applicability of this proposed method in clinic. This work provides an antibody-free, highly selective, and sensitive method for simultaneous detections of L-AβO and L-AβO in real CSF samples, which is significant for the early diagnosis and prognosis of AD.