Abstract: Ba_1-xLa_xAl₂Si₂O₈(x=0, 0.005, 0.01, 0.015, 0.02) were synthesized by solid state reaction. Based on the first principle calculation, the replacement position was discussed. The formation energy of different sites was compared, and the result shows that it is most likely that La³⁺ replaced Ba²⁺. The crystal structure and microwave dielectric properties of Ba_1-xLa_xAl₂Si₂O₈ were investigated. The result shows that La³⁺ can effectively promote the transformation from hexacelsian to celsian, when x=0.02, the conversion ratio reaches 98.71%, and the quality factor (Q×f) value and the temperature coefficient of resonant frequency (τ_f) are increased. when x=0.01, the Ba_0.99La_0.01Al₂Si₂O₈ ceramics sintered at 1400 ℃ exhibits the best dielectric properties: Q×f = 25 674 GHz, ρ=3.30 g/cm³, ε_r =6.938, τ_f = −42.79×10^–6/ ℃.