The present invention provides a portable prefabricated flat bench, wherein, a plurality of brackets are separated from one another with a fixed distance along longitudinal direction and transverse direction and are connected with one another through connecting panels. Each bracket is provided with a seat and supporting legs which are integrally connected with the bottom surface of the seat. The supporting legs gradually expand from the base with a trumpet shape. Each bracket panel is placed on the seat of each bracket. Two adjacent brackets in the brackets are connected with each other through a first connecting panel in the connecting panels. The areas on the prefabricated flat bench, which are surrounded by the brackets that are connected by the connecting panels, are occupied by the second connecting panels in the connecting panels. The portable prefabricated flat bench is easily carried and stored because of small dimension, and can be easily assembled and detached. Furthermore the size of the portable prefabricated flat bench can be adjusted freely.