Phase mismatching for third-harmonic generation (THG) in barium metaborate (BBO) crystal is investigated in detail. Upon using BBO crystal in the Type I (oo→e) scheme, in the two independent planes (principal section of the crystal and the plane normal to principal section), when the input second-harmonic beam deviates from the expected direction, phase mismatching occurs and the angle deviation produces different effects on the conversion efficiency. We numerically simulate these two cases of phase mismatching and identify the relation between the conversion efficiency and the deviation angle in the air. The computational results indicate that the phase matching tolerance in the principal section is 0.2° (in the air), while the phase matching tolerance in the plane normal to principal section is 4.5° (in the air). After conducting a lab experiment, the results agree perfectly with the computational results, indicating that the deviation angle in the principal section has a greater effect on the conversion efficiency.