During freeze-thaw cycles, significant migration of soil moisture, heat, and salts occurs, which exacerbates soil salinization, thereby having a profound impact on agricultural production stability and the sustainability of soil fertility. Based on field experiments, biochar and straw were applied to the 0~15cm soil layer (BQ and CQ) and the 15~30cm layer (BS and CS), with a blank control group (CK) as a comparison. The moisture content, temperature, and salt concentration in the 0~15cm, 15~30cm, and 30~45cm layers were monitored during the freeze-thaw period to investigate the effects of biochar and straw applied at different depths on soil moisture, heat, and salt dynamics. A structural equation model was used to analyze the relationships between moisture, temperature, and salts across different soil layers. The results showed that during the experimental period, the application of biochar and straw significantly improved the water, heat, and salt characteristics of the soil. Specifically, the average moisture content in the 0~45cm soil layer for the BQ, BS, CQ, and CS treatments was increased by 2.85, 3.13, 1.56, and 2.15 percentage points, respectively, compared with that of the control group. All treatments effectively increased soil temperatures and reduced temperature fluctuations during the freeze-thaw period. The average salt concentration in the 0~45cm soil layer for the BQ and BS treatments was increased by 0.34g/kg and 0.40g/kg, respectively, compared with that of the control group. Furthermore, the application of biochar effectively suppressed salt migration by adsorbing salts. The structural equation model results indicated that moisture migration affected both heat transfer and solute movement, and the application of biochar and straw changed the correlations between water, heat, and salts across the different soil layers. These findings can provide theoretical and technical support for regulating soil ecological environments in regions with seasonal frozen soil.