Inclusion Criteria:  Histologically confirmed grade 1, 2, or 3A FL; Patients received prior anti-lymphoma treatment; At least one evaluable lesion according to 2014 Lugano criteria; Age 18 years or older; Eastern Cooperative Oncology Group (ECOG) of 0-2; Life expectancy > 3 months; Able to participate in all required study procedures; Proper functioning of the major organs:  Exclusion Criteria:  Patients who required warfarin or had a history of stroke or intracranial hemorrhage within 6 months, active transformed disease; Histological transformation of follicular lymphoma; Known central nervous system lymphoma; Received a prior allogeneic hematopoietic stem cell transplant. Prior autologous hematopoietic stem cell transplant is allowed; Subjects who have received prior treatment with ibrutinib, or other BTK inhibitors; Uncontrolled active infection, with the exception of tumor-related B symptom fever; Prior nitrosoureas within 6 weeks, chemotherapy within 3 weeks, therapeutic anticancer antibodies within 4 weeks, radio- or toxin-immunoconjugates within 10 weeks, radiation therapy or other investigational agents within 3 weeks, or major surgery within 4 weeks of first dose of study drug; Subjects who progressed or become refractory while on treatment with PI3K inhibitors are excluded. However, subjects who were responding to PI3K inhibitors, but had treatment discontinued due to toxicity, are eligible; Patients require treatment with strong CYP3A inhibitors; Clinically significant cardiovascular disease such as uncontrolled or symptomatic arrhythmias, congestive heart failure, or myocardial infarction within 6 months of Screening; Patients with active hepatitis B or active hepatitis C. Patients who are positive for hepatitis B Surface Antigen (HBsAg) or hepatitis C Virus (HCV) antibodies at screening stage must pass further detection of hepatitis B Virus (HBV) DNA (no more than 1000 IU/mL) and HCV RNA (no more than the lower limit of the detection method) in the row. Hepatitis B carriers, stable hepatitis B (DNA titer should not be higher than 1000 IU/mL) after drug treatment, and cured hepatitis C patients can be enrolled in the group;