Precise, portable, low-cost sample injection is strongly demanded for use in point-of-care testing devices in resource-poor settings; however, current microfluidic sample injection techniques are often expensive, bulky and electricity-powered. To address this issue, we propose a novel syringe flow-stabilizer for hand-powered, precise, continuous-flow microfluidic sample injection. Our syringe flow-stabilizer applies the principle of passive flow-resistance compensation to stabilize the unstable sample flow and has the special advantages of easy-to-use, simple structure, low cost and high stability. The flow stabilizing performance of the stabilizer is characterized via a series of experiments and the results show that our stabilizer is capable of outputting a constant flow rate up to several milliliters per minute under a low threshold pressure. Finally, the fabricated syringe flow-stabilizer is integrated with an inertial microfluidic cell concentrator for high-throughput continuous concentration of trace blood cells from large-volume biofluids. The use of our stabilizer makes the concentration performance totally independent of operation. We envision wide applications of our syringe flow-stabilizer as a hand-powered sample injection unit in various point-of-care testing devices in resource-poor settings.