Cerebral autosomal recessive arteriopathy with subcortical infarcts and leukoencephalopathy (CARASIL) is a rare hereditary cerebral artery disease. The HtrA serine protease 1 (HTRA1) gene has been identified as the causative gene of CARASIL. Here, we report a novel mutation in the HTRA1 gene in a CARASIL pedigree and explore its pathogenesis at the protein level. Subcutaneous tissue biopsy and HTRA1 gene analysis were performed in a CARASIL patient, and HTRA1 and TGF-β1 protein expression in subcutaneous tissue and cultured fibroblasts from the proband were detected by immunohistochemistry and western blotting. A 28-year-old male proband and his brother experienced recurrent stroke, hair loss and low back pain. Abnormalities in the proband were found in the elastic plate of subcutaneous small arteries, and a novel homozygous frameshift mutation (c.161_162insAG), leading to the formation of a stop codon 159 amino acids downstream of the insertion (p.Gly56Alafs*160) was detected. Reduced HTRA1 protein and increased TGF-β1 expression were detected in subcutaneous tissue and in cultured fibroblasts. A frameshift mutation in the HTRA1 gene detected in a CARASIL pedigree resulted in reduced HTRA1 protein and increased TGF-β1 expression, which may cause severe CARASIL and peripheral small arterial disease.