A lattice self-reconfigurable modular soft robot based on the expansion-contraction motion rule is designed, which is composed of several soft modules, each of which is composed of a silica gel main body with positive hexahedron configuration and a master-slave docking surface. The internal bulged design makes it have a good expansion performance. The master-slave docking surface is composed of an iron disk and a suction disk type electromagnet connected with the silica gel main body by thread composition. Based on the relationship between the volume change of the soft module and the internal pressure, the expansion of the soft module is analyzed. The mapping relationship between the inflation pressure and the expansion of soft module is established. Besides, the inflation pressure required for the connection of adjacent two soft modules is obtained. Each soft module can expand 1.5 times under the working pressure of 30 kPa, and the docking and separation of two adjacent soft modules are realized by using the electromagnet connection and the expansion-contraction motion rules of soft modules. The self-reconfiguration of the modular soft robot can be realized by the sequential docking and separation of multiple adjacent modules. The feasibility of self-reconfiguration of soft robot is verified by the self-reconfiguration experiment.