Water scarcity in semi-arid regions intensifies interspecific competition in intercropping systems, thereby constraining agricultural productivity. Although film mulching effectively conserves soil moisture in monoculture systems, its potential to enhance intercropping performance under water-limited conditions remains unclear. This study evaluated whether film mulching could improve soil moisture and thereby enhance photosynthetic performance, ultimately increasing the productivity of maize-soybean intercropping in semi-arid Northwest China. A two-year field experiment compared film-mulched maize-soybean intercropping (FM) with nonmulched intercropping (NM), maize sole cropping (MS), and soybean sole cropping (SS). Soil water content (SWC), crop photosynthetic characteristics, aboveground biomass accumulation (AGB), system productivity, and economic performance were comprehensively assessed. Film mulching increased SWC by 8.12-46.37% in maize strips and 12.47-35.82% in soybean strips relative to NM, predominantly in the 0-30 cm layer. This improvement elevated net photosynthetic rates by 21.09-37.33% and subsequently increased aboveground biomass across both crops at reproductive stages. Consequently, FM substantially improved crop yield and water productivity over NM, achieving land equivalent ratios of 1.30-1.50. Despite 13.80% higher production costs, FM increased total revenue by 39.08-50.16% over NM, resulting in substantially greater economic profit. Path analysis confirmed that film mulching alleviated interspecific water competition by improving soil moisture. The enhanced water availability promoted photosynthetic capacity in both crops, ultimately leading to higher economic returns compared with non-mulched intercropping. These findings support integrating film mulching with strip intercropping as a viable strategy for sustainable agricultural intensification in water-scarce drylands.