Currently, road surface smoothness in China is typically measured using manual detection methods, which are time-consuming, labor intensive, and risky. In addition, these methods do not provide real-time measurements during road construction. To address these issues, we propose a road surface smoothness detection method based on line lasers. This method uses a custom-built line-laser system to achieve high-precision three-dimensional (3D) measurements with millimeter-level accuracy. The issue of deviations in the detection results caused by equipment vibrations during operation is optimized using a wavelet transform algorithm. The comparative experimental results show that the proposed method has over 85% similarity with the flatness detection results obtained from a laser section analyzer on the same road section, with a Euclidean distance of approximately 2. This line-laser-based method can capture the 3D morphology of a road surface, reduce cost losses, and enhance the detection capabilities of roads. The proposed method also provides a comprehensive, accurate, and multidimensional approach to road surface flatness detection for road construction in China.