Abstract: An expandable polymer lost circulation material (LCM), SDP, was developed to deal with mud losses into fractured formations. SDP was made from oligomer resin monomer which reacts at high temperatures with a crosslinker and a toughener under the action of a catalyst. SDP is pressure bearable and its expansion is triggered by temperature. When entering into a loss channel, SDP makes itself adaptive to the space where it is and bridges the space, thereby stop mud losses. The SDP developed has compressive strength under ambient temperature of greater than 71.62 MPa, and compressive strength under high temperature of greater than 17.21 MPa. At temperatures triggering the expansion of SDP, the expansion lag time is 40.07-53.43 min. As temperature increases, the lag time becomes shorter accordingly. LCM made with SDP of different particle sizes can stop mud losses through expansion and bridging. The pressure bearing capacity of a 9% SDP LCM slurry is greater than 9.5 MPa, as laboratory experiment indicated. Application of an LCM slurry made by mixing SDP particles, rigid particles, elastic particles and flexible fibers on the well Xing-1 in Ningxia solved the repeated losses of mud into Yan'an formation, a fractured coal zone. The mud loss was 100% stopped at the first try, and the total volume of mud losses was reduced by 77.67%.