The invention discloses an externally suspended formwork and an assisting device for oblique pile construction. The externally suspended formwork comprises a bottom formwork body, a middle formwork body and a top formwork body, wherein the bottom formwork body, the middle formwork body and the top formwork body are matched with one another. The bottom formwork body is rectangular. The upper portion of the middle formwork body is of a sunken structure. The lower portion of the top formwork body is of a protruding fan-shaped structure. The assisting device comprises a flow guiding chute, a fixed support, an arch-shaped hook frame and an opposite-pull threaded rod. The invention further discloses a construction method for oblique pile construction. The construction method comprises the steps that the flow guiding chute is installed on a fixed formwork; the bottom formwork body is erected; the middle formwork body is erected; the top formwork body is erected; the arch-shaped hoop frame and the opposite-pull threaded rod are installed; the opposite-pull threaded rod and the arch-shaped hoop frame are removed; and the flow guiding chute is removed, and the top formwork body, the middle formwork body and the bottom formwork body are removed in sequence. The problems that the concrete pouring process of a pile opening of an oblique pile is difficult, the pouring effect is poor, the fastening effect between the externally suspended formworks as well as between the externally suspended formwork and the pile opening is not ideal, according to a traditional construction method, the overall effect is poor, the quality is hard to guarantee, and the consistency of the oblique piles is poor are creatively solved.