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Background In order to study the effects of spatial distribution and yearly migration variations of soil water and salt in coastal saline land on photosynthetic production and yield formation of cotton, spatial distribution characteristics of water content, salinity, and pH in soil at 0–200 cm depths in 3 cotton fields that were similar in locality but differed markedly in degree of salinization were determined in April through October, and photosynthetic characteristics and photosynthate accumulation of cotton were also determined. Results The study shows that, the slightly salinized cotton field had lower soil salinity and pH, where soil water content was lower in rainy season (July–August), and at the late reproductive stage (September–October), soil water content was markedly higher than that in the moderately salinized cotton field, where cotton suffered smaller salt-alkali stress, photosynthetic production matched well with hydrothermal resource, the sink organ had a long photosynthate accumulation time and was at the active material accumulation stage for a long time; salt-alkali stress to the moderately salinized cotton field was relieved in rainy season, but at the early reproductive stage (April–June) and the late reproductive stage of cotton, salt-alkali stress remained evident, and photosynthetic production fit more poorly with the rich photothermal resource stage; the severely salinized cotton field subjected to prolonged high salt-alkali stress resulted in low levels of photosynthetic production capacity and yield. Conclusions For the severely salinized cotton field, salt inhibition and other relevant agronomic actions should be taken with greater efforts; for the moderately salinized cotton field, water and fertilizer management should be tightened at the early reproductive stage of cotton; for the slightly salinized cotton field, high-quality efficient cotton production should be developed.