Abstract: A flow cytometer and a fluid system are provided. The fluid system comprises a flow cell, a sample providing unit, a waste container, a sheath container, a negative pressure source, a quantitative unit and a sample flow monitoring unit, the negative pressure source, the waste container and the flow cell are connected, a negative pressure source, which provides a negative pressure relative to the sample providing unit for the flow cell so that the sample providing unit causes the sample to flow into the flow cell under the negative pressure, a sample flow monitoring unit monitors a flow of the sample fluid transported from the sample providing unit to the flow cell and outputs a feedback signal reflecting flow changes of the sample fluid in real-time; wherein the controller receives the feedback signal and controls the quantitative unit to adjust a flow of the sheath fluid according to the feedback signal.