Quick construction method for deep foundation pit accessory structure

The invention discloses a quick construction method for a deep foundation pit accessory structure. An originally-constructed underground diaphragm wall is arranged on the side, close to a main body, of a deep foundation pit to be excavated, and newly-constructed guard posts are arranged on the three other sides. The method comprises the following steps that 1, a first-layer earthwork, a second-layer earthwork and a third-layer earthwork are excavated downwards in a layered mode, a first support, a second support and a third support are erected, an accessory structure bottom plate is constructed, and a bottom plate first-time changing support is installed; 2, a medium plate first-time changing support and a top plate first-time changing support are installed from bottom to top; 3, multiple first-stage underground diaphragm walls are removed from top to bottom, and a bottom plate second-time changing support, a medium plate second-time changing support and a top plate second-time changing support are installed