The optimization of a batch-type quartz crystal microbalance (QCM)-precipitation sensor measuring acetylcholinesterase (AChE) activity was conducted. To covalently bind AChE onto the gold electrode of a QCM surface, glutaraldehyde cross-linking to a cystamine self-assembled monolayer was tried at different cystamine concentrations. At the optimum conditions of the QCM-precipitation sensor, 0.1 M potassium phosphate buffer (pH 8.0), containing 0.01% Tween 80, was used as the reaction buffer, with the enzyme amount of 5 units for immobilization and the substrate concentration of 50 mg/ml. The current biosensor might find a future applicability to the sum parameter detection on organophosphorus and carbamate pesticides.