A repetition rate laser amplifier is the key component of a repetition rate laser; here, the repetition rate of the amplifier is determined by precisely controlling the heat produced during pumping and amplifying. To achieve a repetition rate laser exhibiting kilojoule output energy, a repetition rate laser amplifier prototype with a diameter of 130 mm is developed based on flash-lamp pumping, a neodymium glass gain medium, and liquid cooling. After ensuring the rational mechanical design and selecting a coolant with suitable properties, the developed amplifier prototype operates at 1 shot per minute and exhibits a double pass small signal gain of 1.3189. At 1053 nm, the PV (peak to valley) of 10-shot average double pass wavefronts for the amplifier prototype are 0.2718λ and 0.3223λ when the temperatures of coolant are 20 ℃ and 30 ℃, respectively.