The slight acidic pH changes during food storage and transportation are often difficult to detect in a timely and accurate manner. There is also an urgent need to establish a portable, accurate, and low interference detection method for real-time monitoring of the quality changes of fresh juice that already has its own color. In this paper, a self-ratiometric fluorescent sensor (DCCA/SA bead) was prepared by combining a modified coumarin and sodium alginate (SA), and was used for the extreme pH monitoring of fluorescent by naked eye colorimetry. The derivative of natural active ingredient coumarin (DCCA) exhibited excellent self-fluorescence properties in the ratio of orange red (630 nm) to blue-green (480 nm) under the excitation, and exhibited good linear changes at extreme acid pH range (2.5-5.0), biocompatibility high sensitivity, and anti-interference ability. Based on the multiple interactions between DCCA and SA, DCCA/SA bead could be used for the monitoring small pH changes of fruit juice and storage stage in a fast response time. In addition, analyzing the RGB values of DCCA/SA bead images under ultraviolet light also successfully quantitatively determined changes in pH values. These results indicated that the self-ratiometric fluorescent hydrogel sensor had great user convenience and point-of-care testing (POCT) to monitor extreme acidic pH for freshness of foods.