Coastal expressways usually show a certain degree of differential settlement resulted from under-consolidated soft soil, eroding sea salt and excavated fish pond and other factors, which has caused serious safety hazards to high-speed vehicles. To explore the deformation law in soft soil subgrade, the relationship among the surface settlement, horizontal lateral displacement and subgrade cracks was analyzed. Based on the measured settlement data, two variable-weight combination forecasting models on the basis of the reciprocal value in square sum and absolute value of error were established. The engineering examples shows that when the cumulative surface settlement difference was larger than or equal to 60mm and the horizontal cumulative displacement was larger than or equal to 30mm, respectively, the reflective cracks are likely to appear on the soft soil subgrade. Moreover, the variable-weight combination forecasting model by using statistical principles to reasonably allocate the forecast errors from the four growth curve models has better fitting effect and robustness than the single forecasting model. And it can meet the actual engineering requirements and provide a practical and effective method for the post-construction settlement prediction in the soft soil subgrade of coastal expressway.