The invention discloses a degradation method for acquiring dalteparin and belongs to the field of dalteparin production. The method comprises the following steps: dissolving heparin sodium in water and then using hydrochloric acid for regulating pH to be acidic and adding sodium nitrite for performing degradation reaction; adjusting the pH of the degradation reaction solution to be alkali, adding sodium borohydride for reacting, and then using the hydrochloric acid for regulating pH and performing ultra-filtration; diluting and then using the hydrochloric acid for regulating pH to be acidic, performing the secondary degradation reaction, regulating the pH of the degradation reaction solution to be alkali after the reaction and then adding sodium borohydride for reacting; filtering, and then using the hydrochloric acid for regulating pH, using an ultraviolet lamp for irradiating the solution and adding ethyl alcohol for precipitating; performing column chromatography on the acquired precipitate and using ethyl alcohol for precipitating the acquired eluant; and dewatering the wet solid and drying, thereby acquiring the end product of dalteparin. According to the method disclosed by the invention, the degradation process is effectively utilized to adjust the self-molecular weight distribution, the loss of the molecular weight modification step is reduced and the yield is increased.