The X-ray free-electron lasers (FELs) have stimulated the growing interest of researchers in different fields. This gives rise to an increasing simulation work of design and optimization of FEL facilities and demonstrations of novel FEL ideas. Most of the multi-dimensional simulation codes in use require large computational resources, while one-dimensional simulation codes can merely give an acceptable description of the FEL amplification process. This paper presents the development of a super-fast time-dependent FEL simulation code, which is mainly designed for seeded FEL and its application on a seed FEL facility. The diffraction factors have been introduced in order to model the effects of radiation field diffraction and drastically simplified the working equations. Meanwhile, a specially designed module for seeded FEL has been added in order to study the laser-electron beam interactions with arbitrarily parameters. The code can also provide a fairly accurate, quasi-real-time assistant tool for online optimization.