As the entering of bacterial endotoxin into blood can cause various life-threatening pathological conditions, the screening and detection of low-abundance endotoxin are of great importance to human health. Taking advantage of signal amplification by target-assisted electrochemically mediated atom transfer radical polymerization (teATRP), we illustrate herein a simple and cost-effective electrochemical aptasensor capable of detecting endotoxin with high sensitivity and selectivity. Specifically, the aptamer receptor was employed for the selective capture of endotoxin, of which the glycan chain was then decorated with ATRP initiators via covalent coupling between the diol sites and phenylboronic acid (PBA) group, followed by the recruitment of ferrocene signal reporters via the grafting of polymer chains through potentiostatic eATRP under ambient temperature. As the glycan chain of endotoxin can be decorated with hundreds of ATRP initiators while the further grafting of polymer chains through eATRP can recruit hundreds to thousands of signal reporters to each initiator-decorated site, the teATRP-based strategy allows for the dual amplification of the detection signal. This dually amplified electrochemical aptasensor has the ability to sensitively and selectively detect endotoxin at a concentration as low as 1.2 fg/mL, and its practical applicability has been further demonstrated using human serum samples. Owing to the simplicity, high efficiency, biocompatibility, and inexpensiveness of the teATRP-based amplification strategy, this electrochemical aptasensor holds great application potential in the sensitive and selective detection of low-abundance endotoxin and many other glycan chain-containing bio-targets.