We propose a pre-patterning and post-oxidation-crosslinking protocol to prepare a humidity-sensing actuator. Fe(0) particles (FeP) are introduced into a sodium alginate (SA) film under a magnetic field (15 mT) to achieve directional pre-patterning, followed by treatment in hydrochloric acid that oxidates FeP into Fe2+, and triggers the crosslinking between Fe2+ and SA, which provides the actuator with differential mechanical tensors and induces controlled response to humidity.