Water is the key factor limiting dryland wheat grain yield.  Mulching affects crop yield and yield components by affecting soil moisture.  Further research is needed to determine the relationships between yield components and soil moisture with yield, and to identify the most important factor affecting grain yield under various mulching measures.  A long-term 9-year field experiment in the Loess Plateau of Northwest China was carried out with three treatments: no mulch (CK), plastic mulch (M_P) and straw mulch (M_S).  Yield factors and soil moisture were measured, and the relationships between them were explored by correlation analysis, structural equation modeling and significance analysis.  The results showed that compared with CK, the average grain yields of M_P and M_S increased by 13.0 and 10.6%, respectively.  The average annual grain yield of the M_P treatment was 134 kg ha^–1 higher than the M_S treatment.  There were no significant differences in yield components among the three treatments (P<0.05).  Soil water storage of the M_S treatment was greater than the M_P treatment, although the differences were not statistically significant.  Soil water storage during the summer fallow period (SWSSF) and soil water storage before sowing (SWSS) of M_S were significantly higher than in CK, which increased by 38.5 and 13.6%, respectively.  The relationship between M_P and CK was not statistically significant for SWSSF, but the SWSS in M_P was significantly higher than in CK.  In terms of soil water storage after harvest (SWSH) and water consumption in the growth period (ET), there were no significant differences among the three treatments.  Based on the three analysis methods, we found that spike number and ET were positively correlated with grain yield.  However, the relative importance of spike number to yield was the greatest in the M_P and M_S treatments, while that of ET was the greatest in CK.  Sufficient SWSSF could indirectly increase spike number and ET in the three treatments.  Based on these results, mulch can improve yield and soil water storage.  The most important factor affecting the grain yield of dryland wheat was spike number under mulching, and ET with CK.  These findings may help us to understand the main factors influencing dryland wheat grain yield under mulching conditions compared to CK.