To safeguard public health and ecosystem safety, precise and sensitive detection of acetylcholinesterase (AChE) activity and organophosphorus pesticides (OPs) is highly desired. Although nanozyme-enabled colorimetry is widely applied, the insufficient catalytic activity and selectivity of nanozymes hinder their performance. Herein, composition-tunable AuPd nanozymes with alloy-ratio-dependent peroxidase (POD)-like activity and specificity are prepared via a simple wet-chemical reduction. Experimental analyses reveal that AuPd alloy nanozymes with a mole ratio of 1:3 possess the highest POD-like activity, with maximum reaction velocities approximately 7-fold and 3.5-fold higher than those of Au3Pd1 and Au2Pd2 nanozymes, respectively. Moreover, the Au1Pd3 nanozyme shows negligible oxidase-like activity, effectively minimizing interference from dissolved oxygen and thereby enhancing the accuracy and sensitivity of colorimetric detection. Theoretical calculations reveal that Au1Pd3 nanozymes possess a high d-band center and are more likely to generate hydroxyl radicals (•OH), thus enhancing POD-like activity in the catalytic reaction. Leveraging the selective blocking of active sites by thiocholine, the resultant Au1Pd3 nanozyme-based colorimetric platform was developed to sensitively and selectively monitor AChE activity. Furthermore, by integrating Au1Pd3 with AChE in a cascade amplification strategy, OPs detection was achieved with high sensitivity, reaching a detection limit as low as 0.216 ng mL-1. Overall, this work presents an efficient approach for engineering nanozymes with elevated activity and selectivity, advancing the application of nanozyme-driven colorimetric biosensors.