A foldable and expandable pet carrier bag. The foldable and expandable pet carrier bag includes a pet carrier bag body, the pet carrier bag body is provided with a first pet door, the pet carrier bag body has a back part, the back part is provided with a foldable expanding part. The expanding part includes a top wall, a bottom wall, a left side wall, a right side wall and a front side wall. The left side wall and the right side wall are folded toward the inner portion of the expanding part, respectively. The bottom wall and the front side wall that overlapped on the surface of the bottom wall are swung and overlapped on the top wall. The pet carrier bag is easy to fold, after being folded, the bottom wall of the expanding part fixed on the pet carrier bag body forms a back cushion.