Based on theoretical calculation of the stored energy and small-signal gain coefficient of the travelling wave amplifier, the characteristics of the output laser were simulated. With the increment of the pumping current, the stored energy and small signal gain coefficient increase rapidly. And amplifier energy extraction efficiency can reach more than 76%. In terms of the output energy, it rises linearly with the increasing of the pumping current. When the pumping current is 80 A, a 798 mJ laser pulse can be obtained. Related experiment is carried out on the basis of theoretical simulation. In this experiment, a pulse with 350 mJ energy, 10 Hz repetition frequency, 10 ns width is used as the seed. The size of the Nd∶YAG crystal rod in amplifier is Φ7 mm×134 mm and the doping concentration is 1.1 at%. The maximum peak power of laser diode (LD) is 24 kW. In order to control the temperature of the crystal working environment, three thermoelectric coolers are chosen with a maximum power of 66 W. Ultimately, a 700 mJ, 10 ns, 10 Hz laser pulse is obtained for single-pass amplification. The beam qualities of horizontal and vertical direction are 7.9 and 12.4, respectively, measured by the beam quality diagnostic instrument M-200S.