In order to ease the growing contradiction between mineral resources supply and demand and realize the effective exploration of the complex terrain region, it is urgently necessary to improve the computational efficiency and interpretation accuracy of the ATEM (Airborne Transient Electromagnetic) system. In this paper, the authors used differential conductance and synthetic aperture algorithm to realize fast imaging for ATEM system. In order to further improve the resolution and interpretation accuracy, the authors made an in-depth study of the key technology of synthetic aperture algorithm. Firstly, the authors provided a method for calculating the optimum aperture, and further verified that the aperture size could significantly influence the imaging effect. Then, by using numerical simulation method, the authors calculated the function expression of optimum aperture based on a lot of model calculation results. Finally, by using a multiple model example, the authors confirmed that the synthetic aperture imaging method can effectively improve the resolution both in lateral and vertical directions. This method is not affected by the impact of the massive air data and also has the characteristics of fast computing speed and can be used for real-time imaging. It can further improve the interpretation precision of the airborne transient electromagnetic method. The research results enrich the ATEM interpretation method and provide an important theoretical basis for improving the prospecting precision.