Considering the reversible formation of cyclic boronic esters between boronic acid groups and cis-diol structures, in this study, we develop a graphene field-effect transistor (GFET) device functionalized with phenylboronic acid carbon quantum dots for the sensitive and real-time detection of sugars containing cis-diol structures. The results demonstrate that the device achieves a minimum detection limit of 10 fM for glucose and sialic acid, and exhibits excellent linear relationships between channel current changes and logarithmic concentrations in the range of 10 fM–0.1 mM, with linear fitting coefficients greater than 0.97. Notably, glucose shows a higher absolute current response, while sialic acid exhibits a larger current change sensitivity, indicating the potential advantage of the device in detecting complex biomolecules. In contrast, the detection results for sucrose molecules without the cis-diol structure, long-chain starch with a spatial structure, and common serum interferents such as ascorbic acid, Na⁺, and K⁺ further show that the device has good selectivity for molecules containing the cis-diol structure and is affected by molecular steric hindrance and molecular stability. Combining the biological functions of glucose and sialic acid, along with the advantages of miniaturization, integration, and rapid response, the developed sensor platform holds promise for flexible, miniaturized applications, providing a practical tool for the real-time monitoring and early diagnosis of tumor biomarkers.