Objective To analyse the application value of CT imaging technology in the diagnosis of lung cancer and mediastinal lymph node metastasis. Methods To retrospectively analyse the general data of 100 patients with lung tumours admitted to our hospital from 2021 to 2023. After surgical pathological examination, there were 75 cases and 25 cases of malignant and benign lung tumours, (including 30 cases and 14 cases of mediastinal lymph node metastasis and lymph node reactive hyperplasia, respectively). All of them carried out energy spectrum CT examination. The accuracy of conventional CT and energy spectrum CT for the diagnosis of lung tumours and metastatic lymph nodes, the CT values of lung malignant tumour group and benign tumours under different keV of enhancement scanning modes, and the quantitative parameters of enhancement scanning of different nature of intrapulmonary foci and lymph node lesions in the venous phase were compared. Results The accuracy of energy-spectrum CT in diagnosing lung malignant tumour, benign lung tumour, mediastinal lymph node metastasis, and reactive hyperplasia of lymph nodes was 96.00%, 88.00%, 86.67%, and 85.71%, respectively, which were higher than that of conventional CT, 86.67%, 64.00%, 63.33%, and 50.00% (P<0.05); compared with the group of lung benign tumour, the group of lung malignant tumour group had higher CT values at 40keV, 50keV, 60keV and 70keV levels (P < 0.05); the standard iodine concentration and slope of the rate of the energy spectrum curve of reactive hyperplasia in lymph nodes had greater differences when compared with metastatic lymph nodes of lung malignant tumours (P < 0.05). Conclusion Energy spectrum CT imaging technology has certain clinical application value in the qualitative diagnosis of lung cancer and mediastinal lymph node metastasis.