To reduce the walk-off angle of the extraordinary third-harmonic ultraviolet wave at 355 nm generated by type II KTiOPO4 and type Iβ-BaB2O4 optical crystals and the Gaussian output beam of a Q-switched Nd:YAG laser, a simple theoretical model was developed based on a rotatable BK7 plate of variable thickness. By rotating the plate up to 35?along the beam direction, we reduced the walk-off angle up to～13%. The same phenomenon is predicted by the model, confirming the performance of the model. It is found that due to the walk-off effect, the intensity profile of the third-harmonic generation beam is slightly degraded. To compensate for the observed phenomena and further reduce the walk-off, we used a combination of a convex lens and an axicon to transform the beam profile of the interacting fundamental and second-harmonic generation waves to the zero-order Bessel–Gaussian form. As a result, the walk-off is decreased to～48.81 mrad, providing～30%relative reduction. By using the same BK7 plate rotated up to 35?along the third-harmonic beam direction, the walk-off angle is further reduced to 38.9 mrad. Moreover, it is observed that the beam profile of the emerged Bessel–Gaussian third-harmonic generation beam remains unchanged with no degradation.