The invention proposes a preparation method for a micro-fluidic chip which is used for detecting ultraviolet - visible absorption, the structure of the chip is designed for biochemical analysis in an ultraviolet - visible absorption detection system. The structure of the micro-fluidic chip is used polymeric material such as synthetic glass (PMMA) as a substrate. A transmitted through-hole step structure is processed in a detection point position designed by the chip and glass which can pass through ultraviolet is implanted in a step position, then, problems that low transmission of ultraviolet light of the micro-fluidic chip in polymeric substrate and short optical length caused by a micron dimension groove. The chip of new type has simple structure, easy process, low cost technics, and is easy to popularize and apply.