Uniform submicron carbon spheres of 100–1000 nm sizes are needed in many, such as battery and medical fields. In this paper, a rapid hydrothermal carbonization method was developed for the preparation of uniform carbon microspheres with adjustable particle sizes in the range of 76–930 nm at 190–270 ℃ and 7 MPa in a microchannel reactor. Glucose was used as a carbon source and the synthesis time was as short as 0.5 min. The window conditions of the carbon microsphere synthesis relevant to the glucose concentration, the reaction temperature, and the residence time were investigated. The synthesized carbon spheres were also deeply studied using XRD, SEM, FTIR, Raman, TEM, and XPS. The submicron carbon spheres synthesized at 250–270 ℃ and 0.5–1 min residence times by using 15–25 wt% glucose concentrations as feed had narrow particle size distribution. Carbon spheres with similar particle sizes could be synthesized at multiple combinations of reaction temperatures, residence times, and glucose concentrations, and the sphericity and uniformity of carbon spheres can be easily adjusted. The preparation method takes a significantly shorter time and is much more efficient than the conventional hydrothermal carbonization method.