The performance of a high power long-pulse ultraviolet-preionized TE CO2 laser with an active volume of 2.48 L utilizing pulser/sustainer discharge technique is investigated in detail. The laser head consists of a plano-concave resonator for non-tunable operation or a grating resonator for tunable operation. Stable volume discharge and high laser pulse energy output are obtained under various mixture pressures of 20-40 kPa and various discharge lengths of 10-50 μs. Typical laser discharge voltage/current waveforms and detailed data of laser output pulse energy and electro-optical conversion efficiency under various discharge lengths and various mixture pressures are given. The laser pulse profiles of the four main transition lines are demonstrated as operated with a grating resonator. When operated under a non-tunable resonator and 30 kPa mixture pressure,the laser generates 11.1 J pulse energy at 50.1 μs discharge length,corresponding a laser pulse power of 220 kW;1.52 kW average power at 31.6 μs discharge length and 150 Hz high repetition rate,resulting in an average pulse energy of 10.0 J and an average pulse power of over 300 kW.