The spectral, temperature, and pressure dependences of the birefringence Δni of LiNaSO4 crystals are studied. Dispersion Δn(λ) is found to be anomalous. The uniaxial compression leads to different in magnitude and sign changes of Δnі. The dispersion effect of Δnі changes is weakening under the influence of pressure sigma ⟘ z and increases in the case of sigma || z. The baric changes of the refractive indices are estimated. The refractive indices of the LNS crystal are found to increase under the action of uniaxial pressure, which may be due to the compression of the crystal and a change in the nature of its interatomic bonds.