A butt joint octagonal frustum type floating production storage and offloading (FPSO) system, wherein a floating body is provided with an upper structural body in an octagonal frustum shape and a lower structural body in a regular octagonal frustum shape; in a combined state, a smaller bottom surface of the upper structural body is fixedly connected with a smaller bottom surface of the lower structural body, to form a junction surface; the axis of the upper structural body and the axis of the lower structural body are positioned on the same straight line; the larger bottom of the upper structural body acts as an upper deck of the floating structure and the larger bottom of the lower structural body acts as a lower plate underwater of the floating structure; the junction surface is a full-load waterplane of the floating structure.