A closed-loop control system for ignition energy of natural gas engine is designed in this paper. The primary current is monitored by the current monitoring module in this system, in order to get the actual dwell time when the primary current increases from zero to the threshold current, as the monitoring signals of primary current. During the dwell time period, the obtained monitoring signals of primary current are utilized as the feedback to the enhanced time processor unit of MCU to determine whether the ignition energy is set within the range of setting value. The differences between the primary current and the actual dwell current are calculated through the con- trol algorithm, and the obtained differences are utilized as the input of the compensation dwell time algorithm to calculate the compensa- tion dwell time. Then the dwell time of the next ignition event is adjusted to realize the closed-loop control for ignition energy. The pro- posed closed-loop control method can realize the real time monitoring and adaptive adjustment of ignition energy. Experimental results show that the closed-loop control system can improve the consistency of ignition energy for various driving channels and control accuracy of igni- tion energy for each driving channel. That the increasing rate of spark plug gap slows considerably slows down the aging of spark plug.