The purpose of this study is to evaluate the safety, tolerability and efficacy of iguratimod in adult subjects with diffuse cutaneous systemic sclerosis.

Inclusion Criteria:

• Diagnosis of systemic sclerosis (SSc), as classified using the 2013 American College of Rheumatology/ European Union League Against Rheumatism classification of SSc.
• Diffuse Cutaneous Systemic Sclerosis (dcSSc) as defined by 2001 LeRoy and Medsger Disease duration ≤ 3 years (defined as time from the first non-Raynaud phenomenon manifestation).
• Agree to use effective contraception during the study period (women of childbearing age).
• Smokers agreed to quit smoking during the study.
• Ability to provide informed consent.

Exclusion Criteria:

• The following drugs have been used within one month before screening: including TNF-α inhibitors (continuous use for more than 14 days), IL-6 inhibitors, abatacept (continuous use for more than 14 days), JAK inhibitors (continuous use for more than 14 days).
• Used rituximab within 3 months before screening.
• SSc with tumor.
• People with various lung infections, asthma or other lung diseases such as bronchiectasis.
• For patients with severe heart, liver, kidney and other important organ dysfunction, the evaluation criteria are as follows: ALT or AST is greater than 2 times the upper limit of normal, or total bilirubin rises twice; CPK>400; Renal crisis, or hypertension of various causes (≥160/100mmHg) is not controlled; Creatinine clearance rate <30ml/min; White blood cell count<3×109/L; Hemoglobin <80g/L; Platelet count<60×109/L; Heart function level III-IV; PaO2<50mmHg in resting state; FEV1/FVC<0.7.
• In the period of acute or chronic infection (not including finger ulcer combined infection).
• A history of peptic ulcer or bleeding within 6 months before screening.
• People with allergies or multiple drug allergies.
• People with mental illness or other reasons who cannot cooperate with treatment.