Abstract: As a new cultivation technique, plastic film mulching has been increasingly used in sweet potato cultivation. Expert studies have shown that plastic film mulching significantly increased sweet potato yield by improving soil environment conditions. However, few studies have related sweet potato root formation at early growth stage to storage root thickening at late growth stage under different plastic film mulching conditions. Thus this study used the 'Jixu23' sweet potato starch cultivar as the test material in three mulching treatments to study the effects of plastic film mulching on soil moisture and temperature and on the formation of tuber roots and yield of sweet potato. The three mulching treatments were no plastic film mulching (CK), transparent plastic film mulching (TF) and black plastic film mulching (BF). The results showed that at the early root differentiation stage of sweet potato (0?20 days after planting), plastic film mulching obviously increased soil temperature by 1.0?6.5 ℃ in the 0?20 cm arable soil layer. The transparent plastic film mulching treatment was 0.6?3.5 ℃ higher than the black plastic film mulching treatment. Meanwhile, plastic film mulching improved relative soil moisture content in 0-20 cm soil layer by 9.97%?18.1%. Also relative soil moisture under the black plastic film mulching treatment was higher than that under the transparent plastic film mulching treatment by 1.2%?5.1%. Despite the differences in the two forms of plastic film mulching, both increased root absorption area and root activity after 10 days of planting. Root activity under black plastic film mulching declined significantly (P < 0.05) after 20 days of planting. After 20 days of planting, plastic film mulching treatments improved the photosynthetic rate of sweet potato. Plastic film mulching increased zeatin riboside (ZR) content in differentiation roots at 20?40 days after planting, black plastic film mulching showed more significant effects (P < 0.05) than transparent plastic film mulching at 20?30 days after planting. Plastic film mulching also improved abscises acid (ABA) content in differentiation roots at 40 days after planting. ABA content was significantly (P < 0.05) higher under black plastic film mulching than under transparent plastic film mulching. At tuber root enlargement stage, plastic film mulching significantly improved initial and mean accumulation rates of dry matter in tuber roots. It also increased the number of storage roots, fresh root weight per plant and harvest yield. Under treatments of transparent film and black plastic film mulching, yield of storage roots of sweet potato increased by 10.38% and 15.91%, respectively. From the above, it was noted that plastic film mulching improved soil temperature and moisture, altered the contents of ZR and ABA to promote root differentiation during the prophase period of sweet potato growth and further promoted the swell of tuber roots during prophase growth. As a result of the above, dry matter distribution occurred earlier and more was allocated to tuber roots. Finally, plastic film mulching increased the yield of sweet potato and black plastic film mulching was better than transparent plastic film mulching.