This is an open-label, Simon's optimal two-stage design, single-arm, Phase II study with first 6 patients in the safety run-in portion. PIK3CAmut advanced triple-negative breast cancer(TNBC) patients who have progressed on at least one prior line of systemic therapy in advanced setting will be enrolled to receive inavolisib plus eribulin treatment. The study is carried out in two stages. In Stage One, 10 patients are accrued. If there are 4 or more responses among these 10 patients and positive recommendation by the safety review meeting based on the evaluation of first 6 safety run-in patients' data, additional 16 patients will be accrued in Stage Two, resulting in a total patient number of 26. Otherwise, the study will be terminated and a report will be prepared outlining the observed data and the rationale for termination.