Cyanine compounds having the general formula I for staining biological samples, wherein R1, R2, X, Y, A1 and A2 are as defined in the specification. These kinds of compounds may show good light illumination stability, have a maximum absorption peak around 640 nm that may not change as a function of ambient temperature, have rapidly increased fluorescence intensity upon binding to nucleic acids to form compound/nucleic acid complexes, and have a light spectrum in the near-infrared region, thereby effectively reducing interference from background fluorescence and increasing the accuracy of the detection when used as a staining agent for nucleic acids in a flow cytometer. The compounds provided can be used as a staining agent for erythroblasts in the blood.