The purpose of this study is to initially observe the safety and effectivity of RC18 in Participants with systemic myasthenia gravis.

Inclusion Criteria:

1. Voluntarily signed informed consent ;
2. Patient diagnosed with systemic myasthenia gravis according to The following conditions.
3. Seropositive at screening for AChR（Acetylcholine receptor）-Ab or MuSK（Muscle specific tyrosine kinase）-Ab;
4. MGFA（Myasthenia Gravis Foundation of America） Clinical classification Ⅱ-IIIb;
5. QMG score≥8,and QMG score>2 in four or more items;
6. Maintained any one of the stable standard treatment programs in the trial protocol.

Exclusion Criteria:

1. Combined autoimmune diseases, such as systemic lupus erythematosus,rheumatoid arthritis, systemic sclerosis and so on,except for hypothyroidism and hyperthyroidism;
2. Abnormal laboratory parameters need to be excluded, including but not limited to:
3. Immunosuppressive agents except agents of standard treatment programs were used within one month prior to randomization.;
4. Use of biological agents for targeted therapy within 6 months prior to randomization;
5. Intravenous immunoglobulin therapy or plasma exchange therapy within 2 months before randomization;
6. Active infection during screening(For example,herpes zoster,HIV antibody positive,active tuberculosis,etc);
7. Currently suffering from active hepatitis or severe liver lesions and history;
8. Diabetics with poor blood sugar control:HbAlc>9.0% or FBG≥11.1mmol/L;
9. Currently having a thymic tumor ,or had a thymectomy within 6 months prior to screening;
10. Received live vaccines within 3 months before randomization,or scheduled to administer vaccine during the trial;
11. Malignant tumor patients;
12. Allergic to human biological preparations;
13. Have participated in any clinical trial within 28 days before randomization,or in 5 times half-life period of the drugs of any clinical trial(taking the longer time).
14. Pregnant , lactating women and men or women who have birth plans during the research;
15. Having alcohol or drug abuse that affect the experimental conditions;
16. Investigator considers candidates not appropriating for the study.