Ammonium dinitramide (ADN)-based thrusters have been successfully used in-orbit, but they possess some problems such as catalyst deactivation at high combustion temperatures and the inability to cold start the engine during thruster operation. Thus, a new thermal ignition method using resistance ignition has been developed for ADN-based liquid propellant. Experimental results showed that ADN-based liquid monopropellant droplets could be ignited by resistance ignition at different pressures. Voltage only slightly affected the ignition delay time, combustion duration, and total reaction time at 150 kPa and 200 kPa. In contrast, increasing the environmental pressure effectively reduced the ignition delay time, combustion duration, and total reaction time. When passed through droplets, the current presented a three-stage rapid decline, entered a stable stage, and finally declined again during droplet ignition and combustion. Furthermore, increasing the environmental pressure could reduce the required ignition energy.