Mixed connective tissue disease (MCTD) is characterized by a combination of clinical features of systemic lupus erythematosus, systemic sclerosis, and polymyositis with elevated antibodies to U1 small nuclear ribonucleoprotein (U1-RNP). MCTD is often accompanied by interstitial lung disease as pulmonary involvement. On the other hand, microscopic polyangiitis (MPA) is a systemic autoimmune disease characterized by the inflammation of small vessels (arterioles, capillaries, and venules) mainly affecting the lung and kidney. MPA is associated with elevated serum anti-neutrophil cytoplasmic antibody (ANCA). Complication of MPA in patients with MCTD is rare. So far, only nine case reports of MCTD complicated by MPA with serum myeloperoxidase-specific ANCA (MPO-ANCA) are available. Here, we describe a 64-year-old male suffering from MCTD with MPA. The patient developed interstitial pneumonia with alveolar hemorrhage accompanied by myositis, scleroderma, and elevated anti-U1-RNP antibody and MPO-ANCA levels with substantial systemic inflammation. Strong immunosuppressive therapy (corticosteroid, intravenous immunoglobulin, and cyclosporine A) ameliorated the myositis, interstitial lung disease, and inflammation, with the decrease of MPO-ANCA levels, despite that severe lung complications are often associated with poor outcomes. In conclusion, MCTD may be accompanied by MPA with alveolar hemorrhage. Severe lung complications may indicate a poor outcome, and therefore prompt immunosuppressive treatment should be performed in such patients.