The mesenchymal stem cells will be derived from human umbilical cord. After the subjects are screened and qualified, random number envelopes will be selected to group 12 subjects into group A, group B, group C and group D at a ratio of 1:1:1:1. The subjects of group A and B will be injected intravenously with 1.5x10E6/kg and 2.0x10E6/kg（according to the weight of subject）mesenchymal stem cells respectively at baseline and every 2 weeks, 4 times is a course of treatment. Subjects will be followed up for evaluation on 15 days, 30 days, 45 days, month 2, month 3 and month 6 after treatment. The subjetcts of group C and D will be injected intravenously with 2.5x10E6/kg and 3.0x10E6/kg （according to the weight of subject）mesenchymal stem cells respectively at baseline and every 4 weeks, 2 times is a course of treatment. Subjects will be followed up for evaluation on 15 days, 30 days, 45 days, month 2, month 3 and month 6 after treatment.

After the subjects are screened and qualified, random number envelopes will be selected to group 12 subjects into group A, group B, group C and group D at a ratio of 1:1:1:1. The subjetcts of group A and B will be injected intravenously with 1.5x10E6/kg and 2.0x10E6/kg（according to the weight of subject）mesenchymal stem cells respectively at baseline and every 2 weeks, 4 times is a course of treatment. Subjects will be followed up for evaluation on 15 days, 30 days, 45 days, month 2, month 3 and month 6 after treatment. The subjetcts of group C and D will be injected intravenously with 2.5x10E6/kg and 3.0x10E6/kg （according to the weight of subject）mesenchymal stem cells respectively at baseline and every 4 weeks, 2 times is a course of treatment. Subjects will be followed up for evaluation on 15 days, 30 days, 45 days, month 2, month 3 and month 6 after treatment. Besides MSCs, moisturizers such as boric acid ointment will be given to all subjects.

The PASI combines assessments of the extent of body-surface involvement in 4 anatomical regions (head, trunk, arms, and legs) and the severity of desquamation (scaling), erythema, and plaque induration/infiltration (thickness) in each region, yielding an overall score of 0 for no psoriasis to 72 for the most severe disease. Participants achieving PASI 75 was defined as having an improvement of at least 75% in the PASI scores compared to baseline.

The PGA is a physician's determination of the participant's psoriasis lesions overall categorized by descriptions for induration, erythema, and scaling.

The DLQI is a simple, participant-administered, 10 question, validated, quality-of-life questionnaire that covers 6 domains: symptoms and feelings, daily activities, leisure, work and school, personal relationships, and treatment. Response categories include "not at all," "a lot," and "very much," with corresponding scores of 1, 2, and 3, respectively, and unanswered ("not relevant") responses scored as "0." Totals range from 0 to 30 (less to more impairment), and a 5 point change from baseline is considered clinically relevant. Least Square (LS) Means in total DLQI score were calculated using Mixed Model Repeated Measures (MMRM) with baseline score as covariate, treatment, pooled center, visit and treatment-by-visit interaction as fixed effects.