Abstract: Abstract: Potato is one of the staple food crops across the agro-pastoral ecotone (APE) in North China under rainfed condition. High and stable yield of potato is of significance for ensuring food security in the APE. The combination of planting date and cultivar should be selected according to different hydrothermal year types due to long potential planting time and high inter-annual variation of hydrothermal conditions during the potato growth period in the APE. To investigate the suitable combination of planting date and cultivar for rainfed potato under different hydrothermal conditions, field experiments were conducted under warm-dry (2017) and warm-wet (2018) year types. Each year included three planting dates and cultivars: early- (April 26/27), middle- (May 15/16) and late (June 02/03) planting combined with early- (Favorita), middle- (Connibeck), and late (Kexin_1) maturing potato cultivars. The experiment results were used to compare the length of the growing period, fresh yield and water productively of potato under different combinations of planting dates and cultivars. The optimal combination of planting date and cultivar was determined according to both high yield and water productivity of potato during different year types. Moreover, the study revealed the impact mechanism of change in planting date and cultivar on yield and water productivity of potato and the yield-determining factors of rainfed potato were investigated by conducting the relationship between yield and main meteorological factors during different growth periods of potato. The study results showed that change of plating date and cultivar had significant interactive impacts on phenology, yield and water productivity. The shortest growing period of potato was only 90 d under late-planting of early-maturing cultivar while the longest growing period of potato reached 138 d under early-planting of late-maturing cultivar. The time of potato growing period had a negative correlation with mean temperature (P<0.01). Mean growing period effective thermal time more than 2 ℃ for early-, middle- and late-maturing cultivars were 1 691.5, 1 772.6 and 1 944.6 ℃·d, respectively. The delay of planting date would accelerate the emergence rate and shorten the time from planting to tuberization for each combination. However, the time from tuberization to maturity did not change significantly with the change in planting date. There were different combinations of planting date and cultivar under different year types. In the warm-dry year, average yield across all the planting dates and cultivars was 6 187.3 kg/hm2. Early-maturing variety 'Favorita' under early to middle planting could alleviate the water stress during potato growing period, and had higher fresh yield and water productivity than other combinations. Average fresh yield across all the planting dates and cultivars was 19 390 kg/hm2. Planting late-maturing variety 'Kexin_1' on middle May could achieve the highest yield and water productivity of 23 103.9 kg/hm2 and 6.8 kg/m3, respectively in the warm-wet year. Optimal combination of planting date and cultivar could reduce the water stress during potato growth period and increase the yield and water productivity of potato under different year types. The correlation analysis of yield-climate relationship showed that precipitation from tuberization to maturity had the highest positive correlation with potato yield (P<0.01), and could explain 87% of the variation of potato yield. Our study could provide a scientific reference for choosing the optimal combinations of planting date and cultivar for potato under different hydrothermal year types.