Lab-on-a-chip has evolved into a powerful analytical tool that combines multiple disciplines for biomedical research. The microfluidic reactor in lab-on-a-chip is regarded as a critical functional device and has a direct impact on the performance of the entire lab-on-a-chip system. The rich and flexible design allows the microfluidic reactor to be used for reactions in a micromixing channel or inside the common microchannel/chamber at a microscale. Moreover, a variety of microfluidic reactors could be utilized in the field of biomedical analysis with a high reaction rate and less reagent consumption. This paper reviewed the delicate design principle of microfluidic reactors (mixing and ordinary reactors). The biomedical applications of mixing reactor (reaction kinetics research and preparation of nanoparticle drug carriers) and the mature applications of other types of microreactor (on-chip polymerase chain reaction, enzyme-linked immunosorbent assay, and nucleic acid hybridization) were introduced in detail. Commonly utilized detection technologies (laser-induced fluorescence, light absorption measurement, and electrochemical) and low-cost on-chip detection strategies were summarized in detail. This review may shed light on the design and application of microfluidic reactor in the biomedical area.