Magnetic helical microrobots driven by low-strength rotating magnetic fields have found wide applications, particularly in biomedical research. The motion dynamics of magnetic helical microrobots is critical for their intelligent control and for performing tasks in complex environments. These microrobots can convert rotational motion into translational motion along their central axis. Many factors contribute to their swimming dynamics, such as the geometry of the helix, the properties of the coated magnetic layer, the viscosity of the fluid, and the wettability of the helix in the fluid. In this letter, we establish a comprehensive dynamic model to analyze the swimming properties of magnetic helical microrobots that consist of a rigid helical flagellum, at low Reynolds numbers. The influence of different designs of this kind of magnetic helical microrobots on their swimming velocity, step-out frequency, and maximum velocity is analyzed comprehensively, providing valuable guidance for the design of helical microrobots for different applications. Our results are supported by a number of experimental studies.