Molecular communication (MC) is a competitive candidate of implementing the long-lasting in-body communication for the body-area medical network, considering its unparalleled advantages over conventional wireless communication in bio-compatibility. Blood vessel is a common topic and the influence of blood flow on the propagation of molecular signals has been studied. However, one of the most significant features was overlooked by all past work-blood is a periodic flow with varying rate, so the channel response varies across time. This is similar to the fast-varying channel in conventional wireless systems, but the molecular condition could be much worse because the channel can vary from symbol to next, so using average flow speed for approximation is far from reality. This paper proposes FlowLink to compensate for the drastic channel variation. FlowLink employs a flow meter and records the real-time flow rate, which accumulates over time and forms a 'flow axis'. By resampling the receiver signal on this flow axis with a fixed flow interval, FlowLink compensates for the varying channel response and restores the coherence time requirement for packet-based communication. FlowLink is evaluated on a vessel-like testbed with a periodic flow mimicking blood, which demonstrates an average improvement in decoding BER of 40% compared to conventional flow-unaware decoder.