The utility model provides an arm pier cap roof beam segmental timbering is chosen to big cantilever, includes longeron support, off -load sand tank base group, ring mound steel stand and diagonal brace group. It has the pilot hole that passes the pier stud to reserve on the longeron support, and off -load sand tank base group encircles the off -load sand tank base who distributes around the pier stud including at least four, ring mound steel stand includes the same steel stand of off -load sand tank base number that number and off -load sand tank base organized, and the bottom mounting of every steel stand is on the off -load sand tank base who corresponds, and the top of its every steel stand is supported on the longeron support, diagonal brace group is including encircleing the diagonal brace that distributes around outermost incline limit position department steel stand at least, and the one end of every diagonal brace articulates on the steel stand that outermost incline limit position department corresponds, articulates on the longeron support in its every diagonal brace other end slant, the utility model discloses not relying on the foundation bearing capacity, also need the trompil not influencing pier shaft structure and outward appearance on the pier stud, be applicable to at various types of pier studs, the pier shaft that is particularly useful for the pier stud becomes high stage.