Intensity-modulated radiation therapy(IMRT) has been shown clinically to increase skin doses for some patients and to decrease skin doses for other patients. A set of IMRT plans for hypothetic targets in a solid-water phantom were designed to investigate the reasons. The results showed that the relative distance between tumor and surface was an important parameter which would affect surface dose strongly. The smaller distance would lead to the more monitor units (MUs), the higher dose at surface and the more probability for severe skin toxicity reaction.