Based on the Stark broadening method and the imaging method, the electron densities of the plasma generated at different pulse frequencies, gap distances and inner diameters of the electrodes are diagnosed. The experimental results indicate that reducing the pulse frequency, shortening the gap distance between the electrodes, and using thinner diameter electrode are all in favor of enhancing the electron density. With the help of the global model, we perform the numerical simulation to explore the factors that influence the variation of the electron density. According to the simulations results, we find that the reduced discharge volume results in the increase of electron density with the increase of pulse frequency. When the gap distance between the electrodes is reduced, although the increased absorbed power and the reduced discharge volume both have an effect on the electron density, the reduced discharge volume plays a decisive role in these two factors. Moreover, using a thinner inner diameter electrode can also reduce the discharge volume, which is of benefit to obtaining the plasma with high electron density.