Abstract: Drip irrigation under mulch, as an important water-saving irrigation technique, has been a common potato production practice in Inner Mongolia. However, the advantage of drip irrigation under mulch has dropped due to over-irrigation or non-timing irrigation in the past few years. A field experiment was conducted in Wuchuan County of Inner Mongolia Autonomous Region to evaluate the optimal irrigation frequency of drip irrigation under mulch with the total irrigation of 120 mm. In the study, irrigation frequencies of "Kexin-I" potato cultivar were one time every 4 days, 8 days and 12 days, respectively. Plant average height, leaf SPAD and leaf area index were measured at different plant growth stages. Also tuber yield, starch content and water use efficiency were measured at harvest. Results showed that the depth of moist soil layer was 0~40 cm under the 8 days irrigation treatment. This corresponded with potato root concentration layer, which favored potato growth. Under this irrigation treatment, average plant height, plant dry accumulation, tuber starch content and water use efficiency were 40.8 cm, 8 683.0 kg·hm^-2 , 13.9% and 11.2 kg·mm^-1, respectively. The values were respectively higher by 3.1%, 8.9%, 37.2% and 26.3% compared with the 4 days irrigation treatment; and by 4.1%, 10.1%, 9.3% and 33.3% compared with 12 days irrigation treatment. The highest tuber yield (35 398.5 kg·hm^-2) was under the 4 days irrigation treatment, which was 12.1% higher than that under 8 days irrigation treatment and then 15.4% higher than that under 12 days irrigation treatment. Potato irrigation frequency of drip irrigation under mulch in Wuchuan area was recommended one time every 8 days due to the higher tuber yield and water use efficiency.