A medium access control method for a cognitive sensor network based on broadcast preamble sampling, a cognitive sensor, CS, executing the following steps in each dormancy-awakening cycle: step 1, spectrum sensing: the CS detects whether primary users are occupying an authorized frequency band; if the primary users do not occupy the authorized frequency band, execute step 2; otherwise, execute step 7; step 2, carrier listening: the CS conducts carrier listening; when a carrier listening result shows that a channel is not occupied, execute step 3; when the carrier listening result shows that the channel is busy and receives a preamble code, if a forwarding condition is not satisfied, execute step 7; otherwise, execute step 6; when the carrier listening result shows that the channel is busy and does not receive the preamble code, execute step 7; step 3: the CS detects the length of a data queue of the CS; when the data queue is not empty, execute step 4; otherwise, execute step 7; step 4, the preamble code is broadcast and transmitted: before transmitting data, the CS firstly transmits the preamble code to establish a communication link with a neighbor node wherein said preamble code comprises hops from the node to a gateway; step 5, data transmission: if the CS receives a response ACK of the neighbor node in the process of transmitting the preamble code, the CS immediately stops transmitting the preamble code and begins to send a data packet to the neighbor node; if the acknowledgement ACK is received after the data packet is transmitted, it indicates that the data packet is successfully transmitted, and execute step 7; otherwise, it indicates transmission failure and execute step 7; step 6, preamble code response: the CS enters a receiving state and sends a response ACK, indicating that the CS is ready to receive data; if the data packet is received, the acknowledgement ACK is transmitted to inform a sending node, and execute step 7; otherwise, directly enter step 7; step 7, dormancy: the CS closes a radio frequency transceiver and simultaneously opens an overtime timer for conducting timing; awakening is conducted again after timing is ended, and execute step 1, wherein: said forwarding condition is as follows: H2<H1; H1 indicates hops from the transmitting node to the gateway, and H2 indicates hops from the node to the gateway. step 1, spectrum sensing: the CS detects whether primary users are occupying an authorized frequency band; if the primary users do not occupy the authorized frequency band, execute step 2; otherwise, execute step 7; step 2, carrier listening: the CS conducts carrier listening; when a carrier listening result shows that a channel is not occupied, execute step 3; when the carrier listening result shows that the channel is busy and receives a preamble code, if a forwarding condition is not satisfied, execute step 7; otherwise, execute step 6; when the carrier listening result shows that the channel is busy and does not receive the preamble code, execute step 7; step 3: the CS detects the length of a data queue of the CS; when the data queue is not empty, execute step 4; otherwise, execute step 7; step 4, the preamble code is broadcast and transmitted: before transmitting data, the CS firstly transmits the preamble code to establish a communication link with a neighbor node wherein said preamble code comprises hops from the node to a gateway; step 5, data transmission: if the CS receives a response ACK of the neighbor node in the process of transmitting the preamble code, the CS immediately stops transmitting the preamble code and begins to send a data packet to the neighbor node; if the acknowledgement ACK is received after the data packet is transmitted, it indicates that the data packet is successfully transmitted, and execute step 7; otherwise, it indicates transmission failure and execute step 7; step 6, preamble code response: the CS enters a receiving state and sends a response ACK, indicating that the CS is ready to receive data; if the data packet is received, the acknowledgement ACK is transmitted to inform a sending node, and execute step 7; otherwise, directly enter step 7; step 7, dormancy: the CS closes a radio frequency transceiver and simultaneously opens an overtime timer for conducting timing; awakening is conducted again after timing is ended, and execute step 1, wherein: said forwarding condition is as follows: H2<H1; H1 indicates hops from the transmitting node to the gateway, and H2 indicates hops from the node to the gateway.