A new theory for describing the ultraviolet (UV) laser generation mechanism by frequency doubling of focused Gaussian beam in nonlinear crystal was presented. The nonlinear crystal used for UV laser generation, such as β-barium borate (BBO), has dramatically small phase matching acceptance angles Δθx and large walk-off angle ρ. Numerical simulation shows the variation of the waist of input Gaussian beam brings change in conversion efficiency and far-field transverse intensity distribution. For validating the theory, we investigated the single-pass conversion efficiency in BBO crystal by using acoustic-optic Q switched 532 nm laser; and took a photograph of fringe structure far-field intensity distribution of the UV beam. Comparison between the numerical simulation and experimental results support our theory and analysis. Our theory provides a useful tool for estimating ultraviolet laser generation processes for low power nanosecond pulsed laser and cw laser.