The invention provides a preparation method of a sodium nitroprusside freeze-dried powder injection. The preparation method comprises steps as follows: A) sodium nitroprusside and injection water are mixed at the temperature lower than or equal to 30 DEG C, the pH value is adjusted to 5.5-7.0, and a sodium nitroprusside solution is obtained; B) the sodium nitroprusside solution is pre-frozen at the temperature of subzero 40 DEG C to subzero 35 DEG C for 0.5-2 h; C) the sodium nitroprusside solution is heated to 0-5 DEG C at the speed of 3-7 DEG C per hour after being pre-frozen and then heated to 35-38 DEG C at the speed of 6-12 DEG C per hour, and sodium nitroprusside powder is obtained; D) the sodium nitroprusside powder obtained in the step C) is subjected to thermal insulation at the temperature of 35-38 DEG C for 2-5 h, and the sodium nitroprusside freeze-dried powder injection is obtained. The thinking set that thorough water removal is favorable for the stability of the sodium nitroprusside freeze-dried powder injection in the prior art is eliminated; and the finally prepared sodium nitroprusside freeze-dried powder injection has excellent stability by adjusting parameters of all steps including pre-freezing, sublimation, thermal insulation and drying.