In order to provide an effective way of reading promotion for public libraries in the context of civil reading, this paper introduced and compared two kinds of cross-age reading modes (teenager volunteers-children, older adult volunteers-children) of American public libraries through field research and telephone interviews. Both modes reflect the philosophy of sharing reading in happiness and focusing on meeting the needs of the community and present the characters of providing one-to-one interactive reading outside family, focusing on early childhood reading literacy development, giving participants full autonomy and focusing on training volunteers. Meanwhile, lacking of volunteers is a common obstacle to both modes, and orderly organization and planning, making reading interesting, and having volunteers dedicated to programs are key to the success. The differences between the two modes are mainly manifested in the reasons for volunteer participation, the need for review, the partners and the effect.