Output pulse characteristics of high-pressure CO2 lasers are theoretically investigated. The dynamical mechanism of pulsed electric discharge CO2 lasers is described by means of six-temperature multifrequency dynamical model, which takes into account the effects of the collision-dependent overlap of rotational lines, hot bands, and sequence bands on the gain spectrum, and non-Lorentzian line overlap. The calculations present the influence of specific input energy or output laser energy on laser build-up time and pulse width at different pressures. Theoretical results are consistent with experimental measurements at 400 kPa, 600 kPa, 800 kPa and 1000 kPa, respectively. The pulse width decreases from 120 ns to 35 ns and laser build-up time from 1000 ns to 400 ns, as output laser energy increases from 250 mJ to 1000 mJ. These data are helpful for the design of high-pressure pulsed CO2-N2-He-CO system.