Abstract: In Bohai Oilfield, all horizontal wells and some directional wells are completed by using high quality sieve tube within borehole in order to control sand. During well work-over operation, if the working fluid system is not anti-sloughing, the system will cause wellbore lose stability, which will lead drilling pipe to stick. In addition, the system will destroy the reservoir structure in the near-wellbore area, which will do great damages to oil production and next well completion. We develop a new kind of antisloughing working fluid system on the basis of old system. The new system has good performance in anti-sloughing, which has been confirmed by laboratory tests and application. The anti-sloughing working fluid system can guarantee that fishing can be conducted smoothly.