Abstract: The air ultrasonic source imager (beamforming method) is widely utilized in detecting partial discharges and gas leaks. Calibration of its minimum imaging sound pressure level (SPL) is crucial for quantitatively assessing the instrument, directly impacting the detection capabilities for minimal gas leaks, operating distance, and partial discharge detection. With beamforming technology advancements, optimizing array size, number of transducers, and inter-transducer spacing has enhanced source identification and positioning accuracy. As a result, under portable array conditions, the detectable minimum imaging SPL has decreased, posing challenges in obtaining quantitative results, and causing comparison and measurement issues in domestic and international products. This study establishes a calibration system for the minimum imaging SPL of air ultrasonic source imagers using a low-background noise measurement system. Three widely-used air ultrasonic source imagers are selected for experiments, simulating acoustic signals from partial discharges and gas leaks with small-sized sources. Calibration distance and environment are determined, and concentric positioning rings are used to monitor imaging localization errors of small sources. A 12-line high-precision laser leveler aids in minimizing signal alignment impact. This method enables the calibration of minimum imaging SPL. The measurement uncertainty is assessed, showing that within the 20–40 kHz frequency range, the expanded uncertainty of the minimum imaging SPL can reach 3 dB (k=2).