A field experiment was conducted to examine the effects of plastic mulching (PM) on the photosynthetic rate, endogenous hormones and root yield of summer sweet potato [I. batatas L. cv. Jishu18 and Ayamuraski (Aya.)]. The results showed that the PM treatments decreased soil bulk density, increased soil porosity and soil available nutrient content, and retained soil water content at a relatively stable level, but no significant difference in soil temperature was observed compared with the control. The leaf chlorophyll content, photosynthetic rate, ribulose 1,5- bisphosphate carboxylase activity, leaf area index, soluble sugar content, and adenosine triphosphate content was higher in the PM treatments compared with the control. In addition, root gibberellic acid, zeatin riboside and abscisic acid content increased under PM treatment. The PM treatment improved the soil micro-environment of sweet potato, resulting in a higher storage root yield and top/root ratio (T/R). According to these results, it is concluded that mulching summer-sown sweet potato is an effective way to increase root yield in northern China, most likely through the modification of the soil micro-environment, photosynthetic capacity and endogenous hormones.