Characterized by their low modulus and high stretchability, soft composites have recently attracted great interest from researchers in related areas. The main objective of the present study is on the fracture property and toughening mechanism of soft composites. Two types of soft composites will be studied: soft elastic foam and the double-network (DN) composite. A theoretical/numerical study is carried out over soft elastic foams. By using the analogy between the cellular structure of foams and the network of rubbery polymers, a scaling law for the fracture energy is proposed for soft elastic foams. A phase-field model for the fracture processes in soft elastic structures is further developed to study the crack propagation in an elastic foam, and results have all achieved good agreement with the scaling law. Simulations have shown that an effective fracture energy one order of magnitude higher than the base material can be reached by using the soft foam structure. To further enhance the fracture and mechanical toughness, the second part of the thesis presents a combined experimental and theoretical study of the DN soft composite, which consists of stacked layers of fabric mesh and 3M VHB tapes. The composite exhibits a damage evolution process very similar to that in the well-known DN hydrogels. The testing results show that the strength and toughness of the DN composite is highly dependent on the composition, and in certain range, the DN composite exhibits much higher mechanical strength and toughness compared with the base materials. A 1D shear-lag model is developed to illustrate the damage-distribution toughening mechanism of the double network composite. The prediction of the model agrees well with the measured properties of the composite in various compositions. The DN composite may also be regarded as a macroscopic model of the DN gel for understanding its structure-property relation.