The invention discloses a preparation method of bisalom, which is simple in operation, low in chlorine content, low in swelling rate, equivalent in phosphate binding amount to the preparation on the market in Japan, avoids amplification effect, and is especially suitable for industrial production. The specific steps are: dissolve N,N,N',N'-tetrakis(3-aminopropyl)-1,4-butanediamine in purified water, add epichlorohydrin dropwise, and control the temperature not to exceed 25°C. After the dropwise addition, heat up to 80°C, react for 9 to 10 hours, then cool down to room temperature, add the reaction residue into purified water, stir, pulverize, wash, and dry; among them, N,N,N',N'‑ The substance ratio of tetrakis(3-aminopropyl)-1,4-butanediamine to epichlorohydrin is 1:2.1-2.3.