Objective: To study the clinicopathological features of small cell invasive urothelial carcinoma and the differential diagnosis of the other small cell lesions of bladder biopsy. Methods: Pathological and clinical data of 1 case of small cell invasive urothelial carcinoma were retrospectively analyzed and relevant literatures from China and abroad were reviewed. Results: Male, 82 years old, cystoscope: in the neck of the bladder 10~12 mucosal protrusions such as mushroom, pedicle was not obvious. Morphological features of bladder biopsy tissue under light microscope were: 1) diffuse and nested distribution of subcutaneous small cell tumor cells in the urinary tract, interstitial proliferation; 2) the cells were small, with short fusiform or lymphoid, very little cytoplasm, the local tumor cell cytoplasm was broad and transparent; 3) the nuclei were round, oval or fusiform, nuclear hyperchromatism and structure were not clear, dual core, nuclear overlap and multinucleated giant tumor cell, prominent nucleoli were not seen; 4) no mitotic figures; 5) local tissue extrusion phenomenon and focal coagulation necrosis; 6) vascular invasion; 7) local dysplasia in urinary tract epithelial basal layer and components of small cell tumor in intrinsic membrane were transitional. Immunohistochemistry, a few cells CK weak (+), the remaining markers were (−); pathological diagnosis: small cell undifferentiated carcinoma. Transurethral resection of bladder tumor, no pathology, mesenteric multiple metastatic tumor nodules were found several months later, the morphology of the tumor cells was similar to that of bladder biopsy, the local tumor cell nuclei deviation, visible nucleoli and pathological mitotic figures. Immunohistochemistry, epithelial markers, p63 and CD44V6 were positive, neuroendocrine markers still were negative. Comprehensive consideration of final pathological diagnosis was bladder small cell invasive urothelial carcinoma with intestinal wall metastasis. Conclusion: The diagnosis of small cell malignant tumor in bladder biopsy specimen was difficult. We should pay more attention to its pathological features. Differential diagnosis should be combined with clinical and immunohistochemical characteristics. When the two can’t provide valuable help, the diagnosis should be based on the morphology of HE slices, and it was also indicated in the report. Nuclear mitotic figures should be with or without in the pathologic diagnosis of urinary tract epithelial carcinoma of the bladder biopsy specimen.