© 2015 by the American Society of Agronomy, 5585 Guilford Road, Madison, WI 53711. All rights reserved. Crop yields have increased greatly due to the use of plastic-film mulching in dryland areas of China at low to mid-altitudes. However, its effect at high altitude remains unknown. We studied the effect of ridge–furrow with plastic-mulching practices on crop yields in semiarid farmland at high altitude (2400 m) from 2010 to 2011. The three treatments were as follows: (i) flat plot with no mulching (CK); (ii) alternating ridges (60-cm wide, 15-cm high) and furrows (60-cm wide) without plastic film (RF); and (iii) alternating ridges (60-cm wide, 15-cm high) and furrows (60-cm wide), with the ridges mulched with plastic film (RFM). Potato (Solanum tuberosum L.) and maize (Zea mays L.) crops were planted in 2010 and 2011, respectively. The RFM treatment did not increase potato yields, but significantly increased the ratio of large tuber to total tuber yields (RBT). The RBTs for CK, RF, and RFM were 44.0, 46.8, and 66.7%, respectively. In 2011, the RFM treatment had significantly higher maize yield than the CK and RF treatments, and maize yield was mainly affected by temperature in May. In conclusion, ridge–furrowing with plastic film did not increase the total yield of potato crops compared with non-mulching treatments, but did increase large tuber yield. The increase in soil temperature in May caused by mulching was important for improving maize yield in semiarid farmland at high altitude.