Objective: The purpose of this study was to report the clinical characteristics and prognosis of spontaneous isolated abdominal aortic dissection (SIAAD) based on the dissection length. Methods: Between March 2012 and September 2023, 159 of 7572 patients with aortic dissection were diagnosed with SIAAD and enrolled in the retrospective study. We proposed a new morphologic classification: extensive SIAAD (e-SIAAD) and focal SIAAD (f-SIAAD), based on whether the dissection length exceeds 50 mm or not. The clinical baseline, computed tomography angiography (CTA) findings, and long-term follow-up of the two types were compared. Results: SIAAD prevalence was 2.1%. Patients with f-SIAAD were significantly older (63.74 ± 10.97 vs. 50.70 ± 10.10 years, p < 0.001), had more atherosclerosis risk factors, arteriosclerosis, and penetrating aortic ulcers compared to e-SIAAD patients. Conversely, e-SIAAD presented more acutely (72.97% vs. 34.12%, p = 0.001), exhibited more frequent symptoms (85.14% vs. 61.18%, p = 0.0037), larger dissection diameters (31.89 ± 10.99 vs. 24.41 ± 11.28 mm, p = 0.001), and greater involvement of the renal and iliac arteries. Treatment involved medical management (30%), endovascular repair (65%), or surgery (2.5%), without significant differences between groups. In-hospital mortality was higher in f-SIAAD (six deaths vs. one in e-SIAAD). During median follow-up of 48 months (range, 6–148 months), mortality was higher in f-SIAAD (70% vs. 90% estimated 10-year survival). Conclusions: SIAAD classification by dissection length revealed significant differences in clinical presentation, CTA characteristics, and prognosis. Focal dissections correlated with advanced age, severe arteriosclerosis, and poorer long-term outcomes, emphasizing the need for tailored management approaches.