The invention discloses a data transmission method and a data transmission system for a smart power grid in a basement environment. The method comprises the steps that when a smart electric meter generates new data, a data transmission node sends a first transmission request to an aggregation node; if transmission is permitted, a first transmission permission command and first transmission frequency band information are returned back to the data transmission node; when the data transmission node receives the first transmission permission command, data is transmitted to the aggregation node, and otherwise, a second transmission request is sent to a relay mode; if the relay node permits to transmit the data, a second transmission permission command and second transmission frequency and information are returned back; when the data transmission node receives the second transmission permission command, the data is transmitted to the relay node; the relay node selects an appropriate frequency band to transmit the data; and the aggregation node collects and compresses the uploaded data, and uploads the data to an outdoor backbone network. According to the method and system provided by the invention, a wireless private network and a public network in smart power grid communication can be coordinated, all-area coverage of the smart power grid can be achieved, and data transmission business of the smart power grid can be met.