A modular warehouse system and its assembly method are provided. A plurality of storage containers is combined to form a combined storage area, which is provided with a track assembly, a bin gripping robot, and a plurality of bins. An internal space of at least one track-switching container is in communication with the internal space of at least one of the plurality of storage containers. The track-switching container is provided with a track-switching device therein, and the track-track-switching device is configured to switch the bin gripping robot in at least one of the storage containers from a current running track where the bin gripping robot is located to a target running track. An internal space of at least one outbound-inbound container is in communication with the internal space of the track-switching container. The outbound-inbound container is provided with an outbound-inbound device for performing goods outbound-inbound operations.