A laser diode( LD) side pumped acousto- optic Q- switched Nd:YAG solid state 355 nm ultraviolet laser is reported by using intra cavity double frequency and sum frequency mixing. The Nd:YAG crystal is sidepumped by a LD, the intra cavity green radiation is generated using a type I non- critical phase matched littium triborate (LBO) and subsequent frequency mixing in two type II LBO crystals. A high power and high repetition rate 355 nm ultraviolet laser is obtained by sum frequency mixing back and forth. The output power of 15.3 W at 355 nm is obtained at the pump power of 939.6 W and repetition frequency of 8 kHz with pulse width as short as 90 ns. The conversion efficiency is 1.63%. The beam quality M2x, M2y is 4.33 and 4.56, respectively, and the degree of power instability is ±2.7%.