A novel mode-filtered light detection method is described in which an unjacketed optical fiber is inserted into a transparent capillary tube and three or more detection channels are set on the capillary side for different distances from the port of the fiber. This new method is the basis of synchronization of separation and analysis, with which a modern multidimensional analysis apparatus will be constructed. For samples of different concentration, the more close to the laser incidence port of the fiber the detector has been set up, the greater the change of the intensity profile of mode-filtered light (IF) that is obtained. Vice versa, if a parameter = I0/In is established and, instead of a mode-filtered light signal, the reversed alteration trend of is found in comparison with the alteration trend of IF, the reason is that the background rapidly lowers with the increasing distance to the laser incidence port of the fiber; moreover, the mode-filtered light signal decreases slowly with it. With the present method and apparatus, glucose and glycerol have been determined, with good reproducibility and stability and a small sample volume. Furthermore, a real sample of glucose injection is measured for a detection volume of 5 L, and an acceptable result is observed.