The present study has verified that silver nanoparticles (AgNPs) can be simultaneously generated and immobilized in the micro-nanopores of regenerated cellulose membranes (CMs) via a simple in-situ synthesis, which can be used for colorimetric detection of mercury ions (Hg²⁺) and hydrogen peroxide (H₂O₂). The findings showed that the high-loose interpenetrating porous structure and abundant hydroxyl groups of CMs can serve as both micro-reaction spaces and immobilized sites for the in-situ synthesis of AgNPs. The Ag@CMs, composed of well-dispersed, firmly-fixed, and quantity controlled AgNPs functionalized CMs, have excellent Hg²⁺ colorimetric detection capabilities with a visual limit of detection (VLOD) as low as 5 nM. It can efficiently complete early-warning and semi-quantitative Hg²⁺ recognition, and exhibits high storage stability for more than 180 days. Moreover, the Ag@CMs exhibited peroxidase mimic behavior and catalyzed the oxidation reaction of 3,3,5,5-tetramethylbenzidine (TMB) by H₂O₂ to produce a blue solution. This work proposes a simple “green” approach for the heterogeneous synthesis of controllable and stable AgNPs immobilized on CMs and explores their application in chemosensing.