The invention relates to an accumulated water monitoring apparatus and an accumulated water monitoring method for a cable joint box. The accumulated water monitoring apparatus comprises a box-shaped module disposed in the cable joint box; a movable block of which the upper part is provided with an optical fiber to be detected is processed in the box-shaped module, and holes for an optical cable to enter and exit are disposed around the movable block and on the side walls of the box-shaped module; and a space below the movable block in the box-shaped module is filled with water swelling polyacrylic resin materials. The accumulated water monitoring method comprises: a, mounting the box-shaped module in the cable joint box, and after the optical cable is connected and mounted to the inside of the joint box, packaging the box-shaped module in the joint box; b, utilizing the super absorbent characteristic of the polyacrylic resin materials, so that when water enters the joint box and is accumulated in the joint box due to reasons of aging of a sealing ring and the like, the polyacrylic resin materials in the box-shaped module packaged in the joint box are water-swollen; and c, performing an OTDR (Optical Time Domain Reflectometer) test on a terminal of an optical cable segment by a maintenance personnel, so that determination on accumulated water in the joint box in the middle of a line can be completed.