The aim of this clinical trial is to determine the safety and efficacy of Leflunomide in treating patients with IgG4-related disease by comparing the outcomes of prednisone and leflunomide combination therapy with prednisone mono-therapy. The follow-up period will be 12 months. During the follow-up period, results of laboratory tests and image examinations, IgG4-RD RI and other parameters which can reflect treatment response as well as adverse effect events will be recorded.