Abstract: Whether the engineering quality of the highway constructed on the soft soil foundation can be guaranteed mainly lies in the correct prediction of settlement of raft ground foundation. Asaoka's method is an early method using observational settle ment to predict further settlement, it is simple and based on theory, so Asaoka's method is essentially different from other methods. This is the reason why.so many scholars study on it. This article analyzes the basic theory and deduces the results of settlement, and make use of the cubic spline interpoliation to deal with the observational settlement of not uniform interval, the results indicate that this method is feasible.