Abstract: When lost circulation is encountered during drilling into big caves or macro fractures, squeezing cement slurry is the commonly used method to control the mud losses. When squeezing cement slurry, a highly thickened supporting fluid shall be spotted into the loss zones to support the cement slurry, making sure that the cement slurry stays in place and sets at the mouth of the cave, thereby stops mud losses. The supporting fluids commonly used presently are high viscosity bentonite slurries which are easy to be diluted by water, and therefore cannot support the cement slurries. To formulate a suitable supporting fluid, a high molecular weight polymer, YSZC-1, was synthesized. An oil base supporting fluid, viscosified when in contact with water, was formulated by mixing YSZC-1 with oil. The viscosity of this supporting fluid is adjustable to satisfy the special needs of field operation. Since this supporting fluid instantly becomes thickened as soon as it is in contact with water, it cannot be diluted with water, and the failure in controlling mud losses into caves can thus be avoided. When in contact with water or water base drilling fluid, the apparent viscosity of this supporting fluid increases exponentially, and to a maximum when the volume ratio of the supporting fluid and water base mud is 1：1. An apparent viscosity of 133 mPa·s was obtained by mixing one part (volume) of supporting fluid with 9 part of water base drilling fluid, rendering the supporting fluid strong resistance to water dilution. When aging for 16 h at 150℃, the apparent viscosity did not decrease apparently. This supporting fluid is resistant to salt contamination to saturation, and to calcium contamination to about 5%. It is compatible with cement slurry, and will not cause the cement slurry to "flash set". This supporting fluid satisfies the needs for controlling mud losses into caves, and maximizes the possibility of successful mud loss control in caved formations.