Runoff loss and soil erosion are the main factors threatening grain production in the corn belt of northeast China (CBNC) and are closely related to the planting pattern, row direction, and practice of straw mulching. A 4-year (2017–2020) field experiment was conducted under natural rainfall conditions from 2017 to 2020 to ascertain the effects of the planting pattern, including continuous corn (C), perennial alfalfa (A), and soybean-corn rotation (S-C); row direction including parallel slope (PS) and cross slope (CS); and practice of straw mulching (marked by the subscript SM) on runoff and soil erosion from sloped farmland in the CBNC. Runoff depth and the amount of soil loss under continuous corn with parallel-slope cultivation (PSC) were higher than those of alfalfa with parallel-slope cultivation (PSA) and soybean-corn rotation with parallel-slope cultivation (PSS-PSC) during 2017–2020. Cross-slope planting (CSA, CSC, and CSS-CSC) reduced runoff loss by 7.8–23.3% and the amount of soil erosion by 15.9–73.7% when compared to parallel-slope planting (PSA, PSC, and PSS-PSC). The CSA treatment generated the least runoff depth and soil erosion amount among CSA, CSC, and CSS-CSC. Straw mulching (PSC_SM and PSS_SM-PSC_SM) decreased runoff by 82.6–85.5% and reduced the amount of soil erosion by 87.9–91.5% relative to not using straw mulching treatments (PSC and PSS-PSC). Therefore, cross-slope cultivation and straw mulching can significantly reduce surface runoff and soil erosion on slope farmland. The CSA treatment and the practice of straw mulching were recommended for soil and water conservation in the CBNC.