The purpose of this study is to evaluate the efficacy and safety of Expanded Umbilical Cord Mesenchymal Stem Cells on patients with moderate to severe psoriasis. Any adverse events related to UCMSCs infusion will be monitored.The primary outcome is the improvement rate of PASI(Psoriasis Area and Severity Index) and treatment response will be computed from PASI before and after UCMSCs infusion.

Psoriasis is an immune-mediated, genetic disease manifesting in the skin or joints or both. Numerous topical and systemic therapies are available for the treatment of psoriasis. Treatment modalities are chosen on the basis of disease severity, relevant comorbidities, patient preference. For moderate to severe psoriasis, phototherapy, systemic therapy and biologic immune modifying agents are recommended, but all of them have some drawbacks or limitations. Until now, no curative treatment is available. Therefore, it is important to find new treatment for psoriasis. Mesenchymal stem cells (MSCs) are a kind of adult stem cells that can differentiate into bone, cartilage and adipose cells. Umbilical cord Mesenchymal Stem Cells were isolated from umbilical cord matrix and were reported to treat moderate to severe psoriasis vulgaris and psoriasis arthritis successfully by case reports. For the mechanism of the disease, involvement of the immune system in psoriasis is now widely accepted. Mesenchymal stem cells (MSCs) are found to have the function of immunomodulation, migration to skin lesions, limitation of autoimmunity. Therefore, investigators supposed that the injection of UCMSCs could be beneficial for treatment of moderate to severe psoriasis.

Inclusion Criteria:

1．moderate to severe psoriasis vulgaris ( PASI > 7 or BSA >10% ) 2．18 to 65 years old 3．written/signed informed consent

Exclusion Criteria:

1. guttate psoriasis, inverse psoriasis or exclusively associated with the face
2. Acute progressive psoriasis, and erythroderma tendency
3. current (or within 1 year) pregnancy or lactation
4. current significant anxiety or depression with the Self-rating Anxiety Scale (SAS) > 50 or the Self-rating Depression Scale (SDS) > 53, or with other psychiatric disorders
5. With history of primary cardiovascular, respiratory, digestive, urinary, endocrinologic and hematologic diseases, which can't be controlled through ordinary treatments. Those who with malignant diseases, infections, electrolyte imbalance, acid-base disturbance. Patients with clinical test results listed below: abnormal serum calcium level ( Ca2+ > 2.9 mmol/L or < 2 mmol/L);AST or ALT 2 times more than normal upper limit; Creatinine and cystatin C more than normal upper limit; Hemoglobin elevates 20g/L more than normal upper limit,or hemoglobin reduction to anemia; Platelet count less than 75.0*10^9/L; White blood cell less than 3.0*10^9/L; Or any other abnormal laboratory test results, assessed by investigators, that are not suitable for this clinical study
6. Patients with malignant tumors, or when they were enrolled with abnormal tumor markers or with other organ dysfunction
7. allergy to anything else ever before;
8. current registration in other clinical trials or participation within a month;
9. topical treatments (i.e. corticosteroids or retinoic acid or Vitamin D analogs ) within 2 weeks; systemic therapy or phototherapy (ultraviolet radiation B,UVB) and psoralen combined with ultraviolet A (PUVA) within 4 weeks; biological therapy within 12 weeks;
10. medical conditions assessed by investigators, that are not suitable for this clinical study.