The quality of the image in ghost imaging with thermal light is of great importance in practical applications. Through theoretical analysis and simulation, we find that the intensity fluctuations of the field can greatly influence the visibility of thermal ghost imaging. According to this, we suggest a new scheme to improve the visibility by appropriately scaling the intensity and variance of the incident thermal field. The influence of this method on the signal-to-noise ratio is also studied. In addition, comparison is made with the high-order correlation ghost imaging, another means adopted in recent years to increase the visibility. Our analysis should help promote our understanding of thermal ghost imaging.