Flowing unsupported liquid thin films are widely used in various fields. Energetic ions and high brightness secondary radiation covering THz to gamma rays will be generated when super intense laser acting on such a film, with the significant advantages of high repetition-rate, low cost and continuous operation. Methods for preparing liquid films include impinging jets, converging nozzle, gas-dynamic nozzle and wire-guided jet, each with its own characteristics, which can be measured by optical diagnosis. White light interference and monochromatic light interferometry are introduced, and an example is given to illustrate these two methods. This paper provides an overview of the preparation and characterization of liquid thin film target, and summarizes its applications and prospect in laser-driven radiation sources as well as laser ion acceleration.