LADA is actually a form of type 1 diabetes, which is caused by autoimmune damage of islet β cells and triggered by environmental factors based on genetic susceptibility. LADA shows some characteristics of type 2 diabetes at its onset, which develops slowly and latently, and easily be misdiagnosed as type 2 diabetes due to slowly β cell function deterioration. This is a multi-center, open-label, 1:1:1 randomized controlled trial to investigate the protective efficacy of saxagliptin and vitamin D3 in LADA patients previously treated with insulin. The study comprises twice screening, the 8-week run-in period and the 104-week treatment period. The first screening is to choose the LADA patients from the diabetes patients whose disease duration < 1 year. After obtaining the informed consent from the patient who is willing to participate the 104-week treatment will enter to the 8-week run-in period and receive the guidance of diet, exercise, insulin, blood glucose monitoring, and patient diary recording, etc. The second screening will find out the eligible patients from the 6-week to 8-week run-in period according to the inclusion/exclusion criteria, then the patients will be randomized to the 104-week treatment period. Subjects will be randomized into three groups through central dynamic randomization: insulin, insulin+saxagliptin, insulin+saxagliptin+vitamin D3. Our previous randomized-controlled pilot study showed that dipeptidyl peptidase 4 (DPP-4) inhibitors could significantly improve islet β-cell function in patients with LADA. The main purpose of this study: evaluate whether saxagliptin (and vitamin D3) with insulin therapy can better protect islet β cell function than insulin alone.