Formaldehyde has high reactivity and a short detection half-life, and is widely distributed in organisms and the environment. Formaldehyde in the normal concentration range can participate in one-carbon cycle to maintain the body’s metabolic homeostasis, and abnormal fluctuations in formaldehyde concentration can induce body lesions to lead to a series of diseases. Real-time measurement of the concentration, duration and location of formaldehyde in living cells and tissues is of great significance for deciphering the physiological or pathological function of formaldehyde and diagnosing and treating formaldehyde-induced diseases. With significant advantages such as high sensitivity, good membrane permeability, real-time in situ analysis, small biological damage, and easy operation, the organic small molecule fluorescent probe is a powerful non-invasive tool that can monitor the concentration and distribution of formaldehyde in cells in real time and space. In recent years, a series of small molecule fluorescent probes have been developed for the detection of formaldehyde in living organisms. In this paper, these fluorescent probes for the visual detection of formaldehyde in organisms are summarized from recognition mechanism and fluorescence opening method these two aspects. The recognition mechanism involve Schiff base reaction, Aza-Corearrangement. And the design and development direction of formaldehyde fluorescent probes are prospected.