Microwave-assisted continuous-flow reactors (MCFRs) are a valuable alternative to conventional reactors for accelerating chemical reactions. However, despite several interesting applications, only little quantitative research has been conducted on the temperature uniformity of the heating load. With water as a common inorganic solvent, a novel MCFR type based on a special ridged waveguide for heating water is studied by optimizing some parameters of micropipes in order to achieve better temperature uniformity. Compared to the original reactor, the standard deviation of the electric field decreased significantly when using the optimized reactor under the same heating conditions while the average electric field density increased. The optimized results were verified by experiments.