The present invention provides a category of cyano-substituted asymmetric cyanine dyes having the following general structural Formula I and its synthesizing method. The cyano-substituted asymmetric cyanine dyes in present invention are easily synthesized and have long emission wavelength, high molar extinction coefficient, high sensitivity, good light stability, high fluorescence quantum yield after binding with nucleic acid, and low cell toxicity, which is beneficial for application as fluorescent dyes and could also be used in the field of identifying nucleic acid molecules, clinical diagnostics, and immunoassay testing etc.