To clarify the effects of the combined application of sludge and natural humus material on maize yield and soil physicochemical properties in soils with different types of salinity–alkalinity, field experiments were conducted simultaneously in soda alkaline soil in Xing’an League, Inner Mongolia, and coastal saline–alkaline soil in Yancheng, Jiangsu Province. Two treatments were established: chemical fertilizer alone (CK) and combined application of sludge and natural humus material (T1). Maize yield, aboveground biomass, harvest index, soil salinity–alkalinity constraints, fertility, and structural properties were systematically measured. Correlation analysis and redundancy analysis (RDA) were further used to identify the key soil factors regulating yield formation. The results showed that: (1) T1 significantly increased maize yield at both sites, although the magnitude of yield increase differed markedly. At the Xing’an League site, maize yield increased from 3.28 t/hm2 under CK to 5.30 t/hm2 under T1, representing an increase of 61.54%. At the Yancheng site, maize yield increased from 8.92 t/hm2 to 9.89 t/hm2, representing an increase of 10.85%. (2) At the Xing’an League site, T1 also significantly increased maize aboveground biomass by 60.28%. In contrast, at the Yancheng site, T1 had no significant effect on aboveground biomass but significantly increased the harvest index. (3) In terms of soil properties, T1 significantly increased soil electrical conductivity (EC), available phosphorus, and water-soluble HCO3? content, while reducing water-soluble Na+ content. At the Xing’an League site, T1 also significantly increased available potassium content, mean weight diameter (MWD), and geometric mean diameter (GMD). (4) Correlation and RDA analyses further indicated that water-soluble Na+, HCO3?, and soil organic matter (SOM) were the key factors driving yield formation at the two sites. Overall, the combined application of sludge and natural humus material improved maize yield in different types of saline–alkaline land, but the underlying mechanisms varied markedly between sites. In soda alkaline soil, yield improvement was mainly achieved by alleviating Na+ stress, improving soil structure, and promoting biomass accumulation. In coastal saline–alkaline soil, however, yield enhancement was primarily associated with increased available phosphorus content and improved dry matter allocation efficiency. This study provides a scientific basis for the resource utilization of sludge and the targeted amelioration of different types of saline–alkaline land.