A laser peening scanning system for large workpieces is developed to apply in large workpieces laser peening and to control the laser spots quality in the process. It is composed of a high-energy nanosecond laser as the laser source, a laser spot adjusting part to control the shape and size of the laser spots, a two-axis galvanometer part to guide the scanning laser and a flexible pre-bending fixture to fix and elastically pre-bend the workpiece. Based on the characteristic of laser peening, the control process of the system is designed. An experiment to verify the laser spot quality control ability of the system is conducted. It is concluded that corrected by the laser spot adjusting part based on a pair of orthogonal cylindrical lenses, the laser spots on the workpiece generally meet the proposed demand and their sizes and shapes are well controlled.