A system for monitoring at least one parameter of a fluid contained in a container includes a measuring device based on near-infrared spectroscopy designed to be submerged in the cited fluid to be monitored and to take measurements of the fluid. The measuring device includes a measuring area. The monitoring system includes a flotation system joined to the measuring device. The flotation system is arranged, during the use of the monitoring system, floating on the fluid to be monitored such that the measuring area of the measuring device is submerged in the fluid at a constant depth with respect to the level of fluid in the container, such that all the measurements taken by the measuring device are taken at the same depth with respect to the level of the fluid.