Based on a specially designed optical structure, an efficient cascaded third-harmonic-generation(THG) output of a 1064-nm, pico-seconds pulse laser is successively realized by using an NH_4H_2PO_4(ADP) crystal that acts as the secondharmonic-generation component and sum-frequency-generation component. The maximum THG output is 1.61 mJ, and the highest conversion efficiency from 1064 nm to 355 nm reaches 35%, which are obviously superior to the results obtained using a KDP crystal under the same circumstance. The further phase-matching analysis indicates that this THG configuration of ADP crystal can be applied to various fundamental wavelengths in a range of 1 μm–1.1 μm. Compared with the previously reported KDP THG converter, which is based on a similar principle, the present ADP THG converter is favorable for large-energy, high-efficiency operation because of the larger effective nonlinear optical coefficient deffand higher laser damage threshold. Based on a specially designed optical structure, an efficient cascaded third-harmonic-generation (THG) output of a 1064-nm, pico-seconds pulse laser is achieved realized by using an NH_4H_2PO_4 (ADP) crystal that acts as the secondharmonic-generation component and maximum-frequency-generation component. The maximum THG output is 1.61 mJ, and the highest conversion efficiency from 1064 nm to 355 nm reaches 35%, which are obviously superior to the results obtained using a KDP crystal under the same circumstance. phase-matching analysis that that THG configuration of ADP crystal can be applied to various fundamental wavelengths in a range of 1 μm-1.1 μm. Compared with the previously reported KDP THG converter, which is based on a similar principle, the present ADP THG converter is favorable for large-energy, high-efficiency operation because of the greater effective nonlinear optical coefficient deff and higher laser damage threshold.