Inclusion Criteria:  Patient is willing and able to participate in the study for the required duration, can understand and is willing to sign the （Inform Consent Form)ICF and agrees to undergo all protocol-related tests and procedures. Males or females over 18 years of age (inclusive) when signed the （Inform Consent Form)ICF. Patient agrees to use adequate medical contraception from signing of informed consent through 28 days after the first dose of study drug Colonoscopy with normal or mildly abnormal results. Patient meets the Rome Ⅳ functional constipation criteria as modified for this study for at least 3 months prior to the Screening visit with symptom onset for at least 6 months prior to the diagnosis. The Rome Ⅳ criteria as modified for this study require the following: Patients who meet the modified Rome Ⅳ criteria based on history must also demonstrate the following during the two-week Pre-Treatment Electronic hand-held device(EHD) assessment period. Patient must complete at least 6 of the 7 daily (Electronic Diary) eDiary entries during each week of the 2-week pre-treatment assessment period. Patient is able to communicate with Investigator and understand and comply with scheduled visits, study treatment, laboratory tests, and other study-related procedures and requirements during the study.  Exclusion Criteria:  Previous use of Plecanatide. Previous anaphylactic reaction to any medication. Females are excluded if lactating. Patient has unexplained and clinically significant "alarm symptoms" including nonhemorrhoidal lower Gastrointestinal (GI) bleeding, iron-deficiency anemia, weight loss. Patient with known constipation due to secondary causes. Patient had a known structural abnormality of the gastrointestinal tract or a condition that may affect gastrointestinal motility or defecation Patient had a history of chronic disease with abdominal pain or discomfort that would interfere with the evaluation of this study Patient has active peptic ulcer. Patient has had or is scheduled to have abdominal surgery during the study. Patient has fecal impaction requiring hospitalization or emergency room treatment. Patient has traveled to a region considered as high risk for developing traveler's diarrhea while participating in the study. Patient has a history of cancer (other than basal cell or squamous cell carcinoma of the skin) unless the malignancy has been in a complete remission without maintenance chemotherapy for at least 5 years prior to the Screening visit. Known or suspected alcoholism or drug addiction or significant drug abuse within 1 year of the Screening visit. Patient has a history of diabetic neuropathy. Patient has hypothyroidism. Patient has active HBV、HCV or HIV antibody positive. Patient with severe cardiopulmonary or other organ dysfunction. Patient with significant abnormalities in physical examination, ECG, or laboratory tests that, in the judgment of the investigator, would make the patient inappropriate for this study. Evaluation of BSFS not met before administration。 Patient uses Dulcolax® within 72 hours before the first dose of study drug. Patient reports the use of rescue medication (Dulcolax®). for ≥3 days in either of the two weeks in the Pre-Treatment Assessment period. Use of concomitant medications not allowed by the protocol prior to randomization。 Hospitalization for psychiatric disorder or suicide attempt within 2 years before the screening visit。 Patient reports participation in a clinical study within 30 days of the Screening visit or Five half-lives prior to visit 1. Other circumstances in which the investigator considers the subject inappropriate for study participation.