The invention discloses application of sodium nitroprusside to cyanobacterial bloom control. The application includes the steps of A, dissolving sodium nitroprusside into tap water to prepare an aqueous solution with the concentration being 20mM; B, evenly adding the sodium nitroprusside solution prepared in the step A into the centralized outbreak area of cyanobacterial bloom according to the volume ratio of 0.5-1% to massively extinct cyanobacteria. The sodium nitroprusside has evident killing effect on the bloom-forming cyanobacteria, and the cyanobacteria inhibition rate of the sodium nitroprusside solution of 200 microM to moderately-severe eutrophicated water (with cyanobacteria density being 1*10<6>cells/L) can reach up to 80%. The sodium nitroprusside slowly releases nitric oxide and is durable in algae growth inhibition. The algae are killed by the sodium nitroprusside slowly releasing the nitric oxide, and algal bloom development and deterioration are controlled. The sodium nitroprusside is high in inhibition rate and nontoxic to other aquatic organisms, and the sodium nitroprusside is widely applied to the medical field, for example, the sodium nitroprusside is used as medical injection.