According to the measurement requirements of the gas-liquid two-phase flow parameters in thermal design and modeling of a nuclear reactor, a laser-imaging-based method for measuring the characteristics of bubble groups is investigated. Specifically, a planar laser is used to illuminate the two-phase flow outside the transparent model core to form a transverse detection cross-section, and the forward light scattering images of the bubble groups in this cross-section are obtained by high-speed photography. Then, the images are processed with a roundness classification method and an optimized graph segmentation algorithm of the fast radial symmetry transform to effectively reduce the area calculation errors introduced by the overlapping bubble images. Finally, the vertical velocity of the bubbles is calculated on the basis of the "fault contour-velocity-morphology" constraint of bubbles. Thus, the proposed method achieves the measurement of the instantaneous cross-sectional void fraction, the imaging of the mean value of cross-sectional void fraction, the three-dimensional morphology reconstruction of bubbles, and the calculation of average particle size distribution.