Solving the disparity between rainwater supply and crop water demand is a fundamental issue in semiarid rainfed agriculture. Over the last decades, the ridge-furrow-plastic-mulching (RFM) farming system has been widely used as an on-site (in-situ) rain-harvesting farming strategy, providing a partial solution to the supply-demand mismatch of rainwater. The off-site rain-harvesting system (ex-situ water cistern) for supplemental irrigation has been little used. We established an integrated ex-situ rain-harvesting system incorporated into maize RFM in a semiarid site of northwest China from 2018 to 2019. Five treatments were designed as: (1) CK-1, flat planting without mulching and irrigation, (2) CK-2, RFM without irrigation, (3) RFM60, RFM with 60 mm irrigation, (4) RFM105, RFM with 105 mm irrigation, and (5) RFM150, RFM with 150 mm irrigation. We found that supplemental irrigation treatments significantly increased grain yield, total biomass, and crop water productivity compared to CK-1 and CK-2 (P < 0.05) across two growing seasons, suggesting that ex-situ rain-harvesting irrigation can significantly promote field productivity based on widely used RFM system. The differences mentioned above were the greatest in RFM105 and RFM150, significantly greater than those of RFM60. No significant difference was observed between RFM150 and RFM105. Greater output in RFM150 and RFM105 was closely associated with improved soil water storage and thermal state in two growing seasons. Particularly, RFM150 and RFM105 harvested the highest soil organic carbon and total nitrogen after the second fallow period. The highest economic benefit was found in RFM105, followed by RFM150 and RFM60, the least in the two control groups. Our findings provided a critical case to solve the rainfall supply-demand mismatch. The ex-situ rainwater-harvesting supplemental irrigation can act as a promising solution to upgrade the current widely-used RFM farming system for better adaptation to climate change in the semiarid region.