Abstract: With Fe₃O₄ magnetic nanoparticles and gold nanoparticles as carriers, acetylcholinesterase (AChE)-linked sensitization biosensor was constructed through the specific recognition of antigen-antibody to determine the content of Cd²⁺ in water. Fe₃O₄ magnetic nanoparticles were functionalized by Cd-antigen, and gold nanoparticles were functionalized by Cd-antibody and modified by AChE. The assembly was formed through competitive combination of functionalized Fe₃O₄ magnetic nanoparticles and functionalized and AChE modified gold nanoparticles. The assembly catalyzed the hydrolysis of substrate acetylcholine (ACh) to produce acetic acid, which changed the acidity of the reaction system and realized the determination of Cd²⁺. Linearity range of Cd²⁺ was found in the range of 0.001-10.0 μg·L^-1 with detection limit (3s/k) of 0.24 ng·L^-1. Tests for recovery were made by standard addition method at the concentration levels of 0.010, 0.100, 1.00 μg·L^-1, giving values of recovery and RSDs (n=10) in the ranges of 91.2%-107% and 3.5%-5.4%, respectively.