Abstract: The difference is usually negligible between equal-strain and free-strain consolidation calculations for soils with vertical drains considering constant well resistance. In fact, vertical drains in silt often have a significant variable well resistance effect, the difference of consolidation degree between equal-strain and free-strain, in which the variable well resistance effect is considered, has rarely been reported in the literature. Therefore, according to large-strain geometry assumptions, considering the non-linear compressibility and permeability of silt as well as the fact that well resistance varies with time and depth, established is a large-strain non-linear consolidation model of vertical drain foundations under free-strain theory taking variable well resistance effects into consideration, and the solution upon the model is obtained. Based on the validation of the model solution, deeply and systematically analyzed are the differences in consolidation behavior of vertical drain foundation between equal-strain and free-strain by various factors. The analysis results show that for ideal vertical drain foundations, the difference of the average consolidation degree between free-strain and equal-strain amounts to 15%. In this case, applying equal-strain assumptions instead of free-strain would cause large errors. For constant well resistance model, the difference of the average consolidation degree between two assumptions is around 8%. For variable well resistance model in practice, the difference between two assumptions is within 5%. Taking into account the variable well resistance effect, the different distribution forms of the initial effective stress cause the difference between the two is within 4%. In the variable well resistance model, the rate of consolidation is faster in case of free-strain than that in case of equal-strain when H \mathord\left/ \vphantom H r_\texte \right. r_\texte > 25 , while the rate of consolidation in case of equal-strain is faster than that in case of free-strain when H \mathord\left/ \vphantom H r_\texte \right. r_\texte < 8 . When H \mathord\left/ \vphantom H r_\texte \right. r_\texte takes value at 14 ~ 15, The difference in the rate of consolidation between two assumptions is within 2%. In this case, the equal-strain assumption can be applied to the consolidation of the vertical drain foundations even if free-strain occurs in practice.