The invention discloses a preparation method of an iron-based amorphous magnetically soft alloy thin strip based on HT200. The alloy thin strip is prepared from the ingredient of (HT200)aBb, wherein atomic percentage of elementary composition ingredients is as following: the a is equal to 94 to 98, the b is equal to 2 to 6, and the a+b is equal to 100; the HT200 is gray iron 200. According to thealloy, the commercial gray cast iron is utilized to replace main raw materials of pure Fe and pure Si in existing commercial production, the defined amount of B element is added, a single-roller meltspinning method is utilized, and the commercial gray cast ion is sprayed on a copper roller under 1000 to 1400 DEG C to prepare the iron-based amorphous magnetically soft alloy thin strip. The alloy disclosed by the invention has excellent soft magnetic property and is more suitable for large-scale production. According to the alloy, only the commercial gray cast iron 200 and the B element are utilized as the raw materials, no precious pure element and pure C and P elements which are inconvenient to practically produce is contained, production cost is effectively reduced (cost is saved by about 1000 yuan for each ton of the alloy), and a production technology is simplified.