In order to study the influence of jet spacing on the hydrodynamic and dilution characteristics of dual jets, the velocity and concentration fields of dual jets were studied in stagnant, crossflow, and wave environments using particle image velocimetry(PIV)and planar laser induced fluorescence(PLIF). The axial and cross-section vertical velocity, jet trajectory, and concentration fields were analyzed when the jet spacing varied between 5D, 10D, and 15D, where D is the inner diameter of the jet nozzle. The results indicate that, in a stagnant environment, the decay of the axial velocity for the dual jets with small jet spacing is slow compared with that of a single jet, while it grows faster with the increase of jet spacing and gradually resembles that of the single jet; in a crossflow environment, the shielding effect and entrainment decrease with larger jet spacing, enhancing the bending and lowering the ascending height of jets; and in wavy conditions, there is a dependent high concentration area in the middle of two jet holes with small jet spacing and contour lines grows to be bulge-shaped with larger jet spacing. To reduce the influence of spacing on initial dilution, it is advised that the jet spacing should be larger than 10D in crossflow and wave environments.