The invention discloses an imidazolium alginate ionic liquid and a synthesis method of the imidazolium alginate ionic liquid. The structure of the ionic liquid is shown in the description, wherein R1 and R3 are an H atom, alkyl (carbon chain length is 1-20) or substituted alkyl; R2 is an H atom or methyl; and [Alg]<-> is an alginate ion. The synthesized ionic liquid is soluble in water, shows a clear and transparent state, effectively gives the characteristics of the negative ion of the alginate radical into full play, and has important application in the fields of material carriers and metal adsorption. The synthesis method is mainly technically characterized in that the raw material is easy to prepare, the cost is relatively low, the reaction condition and the reaction process are environmental-friendly, no substances polluting the environment or being harmful for the human body are produced, the synthesis method is simple, the product is pure, no byproducts are produced, the purification process is simple, and the synthesis method is suitable for large-scale batch production.