The influence of external factors from the environment (such as sample matrix, quenching agents, etc. ) and instruments can be reduced by the employment of ratiometric fluorescent probes to obtain more reliable detection results. In this article, two kinds of carbon dots (CDs) are synthesized and used as ratiometric fluorescent nanoprobes, which emit fluorescence at 440 nm and 570 nm, respectively, with 365 nm UV light excitation. The two kinds of CDs have opposite fluorescent responses to Cr (VI). The fluorescent probes are coated on the tip of the designed fiber tip with hydrogel to accomplish the online Cr ( VI) detection. The tooth-shaped structure on the fiber tip, which is processed by a femtosecond laser micromachining system, is beneficial to the excitation of fluorescent probes and the fluorescence collection of the sensor. A low-cost LED lamp is utilized as the excitation light source to save the cost and reduce the size of the sensor. The disadvantage of poor stability of the LED light is overcome due to the use of ratiometric fluorescent probes. In consequence, the miniature sensor realizes the self-correcting. It is proved that even though the intensity of the light source fluctuates greatly, the relative standard deviation (RSD) of 7 consecutive measurements is only 3. 1% . In the range of 0~ 200 μM, the sensor has a great linear relationship for Cr (VI) detection with a detection limit (LOD) of 0. 9 μM. The practicability and reliability of the sensor are demonstrated by the determination of Cr (VI) in real-world water samples.