Abstract: Existing communication technologies of underground wireless sensor network such as ZigBee, ultra wide band, WiFi and so on have some shortcomings, for example, transmission distance is short, transmission rate is low, or coverage and access devices are limited. However, IEEE 802.11ah protocol meets requirements of transmission range, power consumption, network capacity and transmission rate of underground wireless sensor network, and can be used for underground monitoring sensor network to transmit periodic monitoring data. For problems of inflexible grouping for sensor nodes and load imbalance among groups existed in standard restricted access window (RAW) mechanism of MAC layer in 802.11ah protocol, a RAW regrouping method based on time delay optimization was proposed, which regrouped all the sensor nodes for three times according to projected transmission time of data packages, so as to decrease node colliding in a RAW group and transmission time delay. In order to realize RAW regrouping, an assignment identifier(AID) group reassignment method was proposed, which redistributed AID to sensor nodes group by group according to regrouped RAW group and made the AID map to MAC of the nodes by use of part nodes disconnecting access point, so as to decrease RAW regrouping time consumption and avoid network performance degradation caused by no data transmission during AID redistribution. The simulation results show that when underground monitoring sensor network adopts the RAW regrouping method, the network has less time delay, larger throughput and lower packet loss rate than the standard RAW mechanisms with sensor nodes increase, and regrouping process has little influence on the network performance.