The invention provides a particle-image-velocimetry-based three-dimension measuring system and measuring method of near-free-liquid-surface gas-liquid two-phase flow fields. An underwater three-dimensional flow field measurement module consists of two high-speed cameras, a 532-nm-wavelength laser device and a corresponding filter lens, a laser light-guide arm, an underwater watertight missile body, and a hydrodynamic diversion aerofoil profile piece. An air flow field three-dimensional measurement module includes two high-speed cameras, a 1064-nm-wavelength laser device and a corresponding filter lens, a laser light-guide arm, a streamlined cabin, and an air diversion aerofoil profile piece. After a three-dimensional velocity vector of liquid and a three-dimensional velocity vector of gasare obtained, a data processing system synthesizes near-free-liquid-surface gas-liquid two-phase flow fields according to a free liquid surface profile to obtain three-dimensional velocity vectors ofthe near-free-liquid-surface gas-liquid two-phase flow fields, so that the hydrodynamic analysis is carried out. According to the invention, the three-dimensional flow field measurement system is designed based on modularization, so that installation becomes convenient; and global rich two-phase flow field information can be obtained.