Alginate hydrogels have properties that are similar to extra cellular matrix (ECM). Thus, their mechanical behavior approximates soft tissue, which makes them desirable for production of tissue-equivalent soft deformable structures. This study aims to determine the synthesis-structure-property relationship for alginate hydrogel with Young’s modulus in the range of 100∼101 kPa, equivalent to that of human lung tissue. Hydrogels are hydrophilic polymers with 3D network. Homogeneous alginate hydrogels are synthesized by direct mixture of sodium alginate and CaCO3, followed by the addition of D-glucono-δ-lactone to initiate in-situ Ca2+ release and gelation. The influence of alginate concentration and molar ratio of the constituent calcium ion to carboxyl group are evaluated while the Young’s modulus of the hydrogel is carefully controlled within the desired range. Elongation testing is performed to determine the initial-stage Young’s modulus of the material. FTIR is used to characterize the influence of synthesis parameters on the chemical bonds. Empirical relations are established between synthesis parameters and the mechanical property. The result of the study will be subsequently used to guide the additive manufacturing of soft deformable structures based on alginate hydrogels.