The invention relates to a clamping and positioning device for a robot welding positioner of a large weldment and a spring parameter selection method. Specific to uncertainty of hole site deviation of a bottom plate of a workpiece, main parameters of a spring are rapidly and conveniently determined through detailed spring stress calculation; and data are recorded and used for data comparison and lookup, and some judgments or conclusions are made. By means of the clamping and positioning device, the spring has an automatic adjusting function and the fixing function of a locking device, accuracy of automatic welding positioning of a robot is ensured, and reliable and fast positioning and clamping of the large weldment and the robot welding positioner are achieved.