A device for measuring solution concentration includes housing, a catadioptric structure, an electromagnetic radiation emitter and an electromagnetic radiation detector. The housing is formed with a detecting part for receiving a solution to be detected. The catadioptric structure is received in the housing, and includes a ray entrance portion, a first total internal reflection part, a second total internal reflection part and a ray exit portion. An accommodation part corresponds to the detecting part. The emitter is disposed at one side of the ray entrance portion, and a ray sequentially passes the ray entrance portion, the detecting part, the solution to be detected, and the first total internal reflection part. Then, the ray is totally internally reflected and converged to the second total internal reflection part, and is reflected again. Finally, the ray passes the ray exit portion and is received by the detector.