A cylindrical reflex triode was designed and directly driven by a four-stage linear transformer driver to generate high intensity pulsed warm x rays. We developed a numerical model of the cylindrical reflex triode and simulated and studied the experimental electron distribution and the radiation characteristics. The working voltage of the cylindrical reflex triode is 220 kV, and the current is about 600 kA. Under the voltage pulse with a rise time of 100 ns, the electron beam spot is uniform, and the duration of the gap without short circuit reaches 200 ns. The x-ray dose is 385 rad (Si), with an irradiation area of 615 cm2 and a uniformity of less than 2:1. The radiation field distribution is basically consistent with the simulation results. Compared with the two-stage series diode on the Flash-II accelerator, the x-ray conversion efficiency of the cylindrical reflex triode is increased about 1.6 times.