To meet the application needs for producing long-pulse electron beams and high-power microwaves, a pulse-forming network Marx generator with a pulse duration of 260 ns is presented in this paper. This generator is composed of 20 stages of pulse-forming network modules. Each module is formed with nine capacitors connected in parallel. The generator functions at 44 kV, which is lower than the rated voltage of the mica paper capacitor, to improve the lifetime. The impedance of the generator is designed to reach 45 Ω. To avoid the strong coupling between the adjacent stages, the physical layout of the generator adopts a zigzag design. The generator is housed in a gas pressurized vessel of 600 mm in diameter and 580 mm in length. Across a 50 Ω load, it can deliver quasi-rectangular pulses with a pulse duration of 260 ns and an amplitude of 500 kV for a single shot. The output pulse features a plateau duration of 160 ns and a leading edge of 45 ns. In burst mode, it can steadily output ten pulses of 450 kV at a repetition rate of 10 Hz on either a resistive load or a diode.