Folding chair armrest locking device and folding chair

A folding chair armrest locking device, comprising:

• a casing having therein an armrest fitting portion and a storage in communication with the armrest fitting portion;
• a rack having an armrest coupling portion and a first dentate portion and being partially disposed in the armrest fitting portion, with the first dentate portion partially facing the storage;
• and
• a locking module having a locking element, a wrench and a resilient component, with a second dentate portion disposed at a first end of the locking element and in the storage and facing the first dentate portion, the wrench having a pivotal connection portion eccentrically positioned and pivotally connected to a second end of the locking element, with the resilient component disposed in the storage and resiliently supporting the locking element, wherein the resilient component pushes the locking element to allow the second dentate portion to mesh with the first dentate portion when the wrench is located at a locked position, wherein, when the wrench pivotally rotates to an unlocked position, the locking element is pulled downward to cause complete separation of the second dentate portion and the first dentate portion, and the wrench abuts against the casing to enable the locking element to compress the resilient component.