The technique of electrospinning has been researched for several decades. Almost all parameters have been investigated in the past years, e.g., solution parameters, process parameters, and environmental conditions. Among the solution parameters, the viscosity of the polymer solution is an extremely important factor for fiber formation and morphology. In general, however, viscosity of the polymer solution is mostly controlled by the solution concentration or by the molecular weight of the polymer in electrospinning field. Herein, we described the reason of a well-known but not completely explained conclusion that the needle diameter can have an influence on the fiber morphology. In this study, polyethylene-oxide (PEO) with a molecular weight of 400,000 g/mol was dissolved in a mixture of ethanol and water with a proportion of 1:3. The relationship between the viscosity of the polymer solution and shear rate was characterized by a plate–plate rheometer. A shear flow model was discussed, while polymer material was flowing through a needle, which presented that different deformation rates were imposed on materials due to variable needle diameters. Combining the rheological experiments and analysis of the shear flow model, the viscosity of polymer solution flowing out the needle was predicted by needle diameter. Through observing the obtained fibers' morphology by scanning electron microscopy and measuring their diameters by image processing software, it was found that the fiber diameter increased with the increase of needle diameter, as expected, which agreed with the relationship of fiber diameter and polymer viscosity.