A transparent tube is used as the fluid pathway of a flowthrough cell, which not only makes sure the fluid can retain the stable laminar state without micro-bubble existing, but also can avoid the turbulent state and the micro-bubbles that produce the negative peak and the interference peak. Utilizing the transparent property of the pipe wall, the outer wall of both ends of the optical pathway is made to be a smooth plane and forms a single unit planar light inlet window with the pipe. The cell does not need to be furnished with a glass window or a quartz glass window any more. This design has solved the leakage and dead angle problems. The invention not only simplifies the structure but also increases the precision.