In order to bring forth a new method to design Wovefront Coding(WFC) system with commercial lens design software,a new merit function formed by the traditional imaging quality factor,Strehl Ratio(SR) and Encircled Energy(EE),was proposed to describe the "insensibility to defocus".A triplet system using WFC technology was designed by the new merit function.The new system extends the depth of focus more 10 times than that of the original system.The results show that the WFC system designed by proposed optimal method can be achieved in design software CODE Ⅴ and zemax and can come to a better trade-off between "insensibility to defocus" and "restorability".