In recent years, picosecond laser in ultraviolet (UV) has manifested great importance for applications both in science and industry, such as biomedical research, micro machining, etc. Now, the well proven approach to generating ultra-short UV pulses is extra-cavity frequency conversion based on nonlinear optical (NLO) crystal, due to the lack of suitable laser sources directly generating UV laser. In this process of harmonic generation, the length of nonlinear crystal is an important factor affecting the conversion efficiency and beam-quality. The optimal length of the nonlinear crystal is influenced by incident laser parameters and crystal absorption coefficient. At present, for the UV 355 nm picosecond laser generated from extra-cavity sum frequency, published are few reports about detailed analysis and research on the influence of photon ratio of the incident laser beams and nonlinear crystal absorption on optimal length of sum frequency crystal. In this paper, the steady-state solutions with the highest conversion efficiency under different incident conditions are obtained by theoretical analysis and numerical simulation of the three waves coupling equations. The effects of different photon ratios and absorption effect of the sum frequency crystal on the optimum crystal length are analyzed. We propose a solution based on the fundamental frequency laser amplified to shorten crystal length and improve conversion efficiency. In this scheme, the 532 nm second harmonic laser with a high conversion efficiency over 65% can be achieved by LiB₃O₅ crystal. After that, the 1064 nm fundamental frequency laser is separated from the second harmonic laser, and then it is amplified by the Nd:YVO₄ laser crystal pumped by an 808 nm laser diode. Finally, the ultraviolet 355 nm picosecond laser is obtained by combining the 1064 nm fundamental frequency laser with the 532 nm second harmonic laser in the LiB₃O₅ crystal. The simulation results show that the incident photon ratio of the sum frequency reaction can be changed by amplifying the residual fundamental frequency laser, and the optimum length of the sum-frequency crystal corresponding to the highest conversion efficiency can be shortened. Meanwhile, the absorption and walk-away effect of the sum frequency crystal can be also reduced. The final 355 nm laser output power can be increased more than 40 percent compared with the traditional scheme of early reports. In consequence, the high sum frequency conversion efficiency of the UV 355 nm picosecond laser can be obtained by changing the photo ratio of the incident laser beams through amplifying the fundamental frequency laser.