Abstract: To address the growing demands for three-dimensional laser scanning in terms of processing range, accuracy, and efficiency, a three-dimensional in-situ laser dynamic focusing scanning system was proposed for the first time in this research. It could achieve a scanning range of 1.5 m×1.5 m×0.4 m, and the focusing spot radii were smaller than 50 μm. By controlling the movement of the dynamic adjustment lens along the optical axis and the centering deflection of the plane mirror along the X and Y axis within a spherical coordinate system, the system enabled in-situ laser focusing scanning in three-dimensional space. The combined design of multiple lenses reduced the divergence angle of light passing through the focusing lenses, significantly enhanced the stability of system and reduced the precision requirements for lens processing, assembly, and control tolerances. Experimental results demonstrated that the system can meet the design specifications, with a maximum focal length exceeding 2.1 m, the root mean square (RMS) radius of the focused spot below 26 μm and having a high roundness. The trajectory of the dynamic adjustment lens and the deflection angle of the mirror were tailored to the working distance of the system, facilitating the control of the system and improving the laser scanning rate. Further analysis of the system's optical and physical propagation, including diffraction energy, wavefront, and illumination, confirmed that the system was operated close to the diffraction limit at each focusing position. The impact of lens processing, assembly, and control tolerances on the system was also analyzed, revealing that the RMS radius of the focused spot across the entire scanning space meet design requirements and maintained good consistency within achievable practical tolerance ranges. Finally, based on the optical design scheme of the system, the corresponding mechanical structure realization scheme and control strategy were proposed. This study provides important guidance for the design of laser dynamic focusing scanning optical system, and has significant practical values for expanding the application of three-dimensional laser scanning technology.