The concentration of formaldehyde in the environment must be precisely monitored, as it is closely linked to human health. In this paper, a decaboryl derivative formaldehyde fluorescent probe (M1) was synthesized for the first time by introducing a 5-amino-isoquinoline group into decaborane parent. Using theoretical calculations, 1H-NMR, 11B-NMR, HR-MS, and FT-IR, the molecular structure of the probe was determined and its response mechanism to formaldehyde was examined. The fluorescence response of the probe to formaldehyde was then tested, revealing an augmented response to formaldehyde in a solution of 0-600μM, with a detection limit of 4.18×10-6M. The results show that the formaldehyde fluorescence probe has the advantages of good linearity, strong anti-interference and high sensitivity. On this basis, a fiber optic formaldehyde fluorescence sensor based on M1/PMMA thin film was constructed in this paper. This fiber optic fluorescence sensor, with its high selectivity, low detection limit, on-line and remote monitoring, and other advantages, was successfully applied to the detection of formaldehyde in both food and aqueous solution, with results that were reliable compared to those of acetone. The detection limit of formaldehyde was raised to 6.9×10-8M.The potential for its utilization in the chemical, biological, environmental, and other formaldehyde detection fields is quite promising.