MFS is a program development system based on rewriting techniques. The language provided by MFS, which called MFSL, is a mixed language that combines enhanced functional language and algebraic speci-fication language. Optimal rules in MFSL can improve efficiency and satisfy specific requirements. Both the type system in MFSL and the mechanism of proving and testing in MFS can help the programmers to find problems early and can improve the correctness of program. The efficiency of the implementation of the system developed by MFS is high due to the techniques used by MFS such as needed parallel outermost reduction strategy,graph reduction and witnessed test set approach.Higher efficiency and correnctness of program can be developed by MFS in a shorter period.