Objective To establish a lung metastasis model based on Lewis subcutaneously-implanted tumors in C57BL/6 mice, and observe the pathological and ultrastructural changes in lung metastasis. Methods C57BL/ 6 mice were subcutaneously implanted with Lewis cells in the armpit of the right forelimb. The tumor volume was measured regularly and a survival curve was generated. Anatomical observations were performed on mice whose survival time was more than 35 days, hematoxylin-eosin (HE) staining was used to detect pathological changes in the lung tissue, and the ultrastructure of the lung tissue was observed by transmission electron microscopy. Results The subcutaneous tumor formation rate of the right forelimb axillary was 100% at 14 days after inoculation, and mortality occurred from 22 days, with a mean life span for the mice of (33. 00 ± 6. 98) days. Anatomically, typical metastases were observed on the lung surface, which had a round translucent appearance and surrounding hemorrhage. HE showed intense staining of the lung metastases, which were nearly round in appearance, with obvious boundaries with the surrounding tissue. The tumor cells were closely arranged and contained abundant capillaries. Under transmission electron microscopy, the tumor cells in the lung metastases showed significant atypia, including meganucleus, dicaryon, and singular nuclei. The cytoplasm of the cells contained a large number of glycogen particles and mitochondria, and cell junctions were evident between tumor cells. Conclusions Prolongation of the survival time resulted in the formation of typical lung metastases in mice with a subcutaneous tumor. The pathological and ultrastructural changes in lung metastases were consistent with the characteristics of malignant tumors.