Birefringence plays a great role in phase matching of the nonlinear optical (NLO) crystals. Small birefringence restricts various crystals from achieving deep-ultraviolet laser output although they exhibit short UV cutoff edges and high second-harmonic generation (SHG) intensities. An access to achieve deeper coherent light output through external pressure on NLO crystal, K₃B₆O₁₀Cl is proposed and demonstrated through computer experiment based on the first principles theory. The “hot spot” in structure that determine the SHG effects and birefringence were highlighted. The shortest achievable phase-matching wavelengths are predicted based on calculated refractive indices. It is found that the quasi-planar (B₆O₁₀)²⁻ group is the dominant contributing unit to optical anisotropy. The pressure-induced increase of polarizability anisotropy of (B₆O₁₀)²⁻ group can notably enlarge birefringence which extends the shortest achievable wavelength of K₃B₆O₁₀Cl frequency conversion. The results show that pressure engineering may be a promising scheme to overcome the drawback of small birefringence of some NLO crystals.