The invention discloses a side plate of a foldable container. The side plate is formed by connecting an inner face plate and an outer face plate. A reinforcer is arranged in the side plate. The reinforcer is an integrated part. At least part of the reinforcer is located in the middle of the side plate. At least the other part of the reinforcer is located at the left end and the right end of the side plate. By adopting the side plate, the stacking capacity of a box is higher, so that the anti-deformation ability is higher while the volume of the box is maximized, and the reliability is improved.