A robot and a collecting method for collecting polymetallic nodules in deep-sea are provided. The robot includes an underwater moving carrier and a collecting module, and the collecting module is fixedly mounted on the underwater moving carrier. The collecting module includes a collecting frame, a collecting pump, a rack and a collecting tube, the collecting frame is installed at the bottom of the rack, and the collecting pump is a piston pump, which includes a piston and a cylinder with open lower-end. The upper part of the cylinder is a collecting area, the lower part is a piston stroke area, the collecting tube is connected to the cylinder of the collecting area, and a check valve is arranged in the middle of the piston.