Abstract: Mud loss is a complex problem frequently encountered in drilling operation. Polymer gel is often used as a lost circulation material (LCM) to control mud losses. Conventional polymer gel lost circulation material has poor high-temperature stability and low-pressure bearing capacity, which are not benefit to mud loss control. A new high-temperature fiber reinforced gel LCM has been developed through molecular structure design using monomers such as acrylamide, butyl methacrylate and 2-acrylamido-2-methylpropanesulfonic acid. A flexible fiber was used as reinforcing material and ammonium persulfate as initiator in the synthesis reaction. The intermediate product was then reacted with a high molecular weight crosslinking agent BWL to produce the final product. The effect of the flexible fiber on the rheology of the gel LCM was studied. The gel LCM was studied, with SEM, thermogravimetric analysis and mud pressured loss control test etc. for its micro structure, thermal stability, swelling performance after water absorption and performance in controlling mud losses under pressure. It was found that the existence of the flexible fiber strengthens the spatial network structure of the gel LCM, making it much tougher. The particles of the gel LCM have good thermal stability and swelling property when absorbing water. When water is absorbed, the gel LCM swells and is squeezed into the channels through which mud is lost to stop mud losses. High permeability loss zones sealed with this gel LCM at 140℃ has pressure bearing capacity of greater than 7 MPa, wide fractured zones sealed with this gel LCM had pressure bearing capacity of greater than 5 MPa, indicating that this gel LCM can be used to control mud losses in high temperature high pressure environment.