The femtosecond laser direct-writing technique has been extensively explored for fabricating three-dimensional optical waveguide devices in transparent, hard, and fragile materials owing to its ultrashort pulse duration, ultrahigh peak power, high manufacturing precision, and efficiency. Recently, there is a growing interest in producing optical waveguide devices in flexible polydimethylsiloxane (PDMS) using the femtosecond laser direct-writing technique. This study reviewed the latest development in femtosecond laser direct-writing Type-I optical waveguide devices in flexible PDMS, focusing on three aspects: the basic principle of different kinds of optical waveguide written by femtosecond laser; materials design, direct-writing principle, and processing technology of Type-I optical waveguide directly-written by femtosecond laser in flexible PDMS in last five years; and research progress of the application of the optical waveguide devices in flexible PDMS based on femtosecond laser direct-writing technique for diffraction gratings and cochlear endoscope. Finally, herein, the research progress in this field is summarized, analyzed and concluded, and the future research, application, and developmental direction are outlined.