Fertilization and early embryonic development as the beginning of a new life are key biological events. Hydrogen polysulfide ( H 2 S n ) plays important roles during physiological regulation, such as antioxidation-protection. However, no report studied in situ H 2 S n fluctuation during early embryonic development because of the low abundance of H 2 S n and inadequate sensitivity of probes. We herein construct a polymeric nanobeacon from H 2 S n -responsive polymer and fluorophores, which is capable of detecting H 2 S n selectively and signal amplification. Taking the zebrafish as a model, the polymeric nanobeacon revealed that H 2 S n level was significantly elevated after fertilization due to the activation of cell multiplication, suppressed partially during embryonic development, and finally keep steady up to zebrafish emergence. This strategy is generally accessible for biomarkers by altering the responsive unit on and significant for facilitating biological analysis during life development.