Abstract: The experimental study of CT imaging was carried out when the activity of radiation source is very low in the case of CT detection for luggage CT inspection system. The method of adding and averaging was used to reduce the effect of statistical fluctuation and noise. And the quality of CT image is improved. Experiments show that for weak signal conditions, using the method of adding and averaging, CT imaging has high quality and can realize property identification requirements. In addition, the A/D quantization error is also analyzed.