Design and experiment results of an all solid-state 355 nm ultraviolet (UV) laser based on Nd:YAG/Cr:YAG composite crystal are reported. The 1064 nm fundamental frequency radiation of high peak power and small waist was generated by plano-plano cavity. Outside the cavity, the fundamental beam was passed through a KTP crystal and a LBO crystal without been focused. The KTP crystal was used for frequency doubling of the 1064 nm radiation to generate second harmonic 532 nm radiation, and the LBO crystal was used as a sum-frequency of the 1064 nm radiation and the 532 nm radiation to generate 355 nm radiation. Although both Nd:YAG and Cr:YAG are isotropic crystals, the 1064 nm fundamental frequency radiation was near-linearly polarized in particular situations. This characteristic is benificial for the conversion of fundamental radiation to second harmonic generation (SHG) and enhance the conversion efficiency of the whole laser system. And divergence angle and bandwidth of the fundamental beam related to conversion efficiency of both SHG and the third harmonic generation (THG) should be within the range of crystal acceptance angle and acceptance bandwidth. Considering these factors, the cavity had been designed carefully. When the laser diode (LD) pump power is 8 W, the peak power of the fundamental beam was 28 kW, and finally, the 355 nm radiation average power of 124 mW was achieved.