This paper examines the decisions underwriters and issuing firms make during seasoned equity offer-ing and the impact of underwriter switch on the price performance of issuing firms． Our theoretical model shows that when there is a severe information asymmetry between the underwriters and issuing firms ( IPO) ， all firms can choose reputable underwriters． As firms and underwriters know each other further and deeply( SEO) ，reputable underwriters can know the real asset of issuing firms and may refuse low-quality firms to maintain their reputation． Therefore，low-quality firms can only choose bad reputation underwriters at the SEO stage． The empirical results are consistent with the theoretical model: low-quality firms have to switch under-writers during the SEO period． There is no significant difference between a switcher and non-switcher during IPO，but underwriter reputation of a switcher is worse during the SEO． Meanwhile，firms managed by former IPO underwriters exhibit lower discount levels and better long-run performance compared to the firms that switch their underwriters，and this result is robust to various checks．