Abstract: To prevent high temperature thickening and maintain good HTHP fluid loss and rheological property of high temperature resistant drilling fluids,a low molecular weight polymer fluid loss additive PFL-L was polymerized by oxidation/reduction reaction,and was analyzed by infrared spectrum and thermo gravimetric analysis.Its molecular weight was tested by gel permeation chromatography,the fluid loss and compatibility in drilling fluid were also evaluated under high temperature and high salinity.Test results show that PFL-L has good thermal stability and polydispersity of molecular weight,is suitable for drilling fluid in ultra-deep well as an additive.PFL-L can reduce fluid loss and control viscosity significantly at high temperature and high salinity,the heat resistance up to 220℃,salinity tolerance to saturation.It was applied in Well Xuwen X-3 which is an important exploratory well of Sinopec.Its field application shows that the drilling fluid has good high temperature stability,and met the demands of long open hole that drilled in the third spud to the total depth of 6 010 m.PFL-L,as an ultra-deep well drilling fluid filtrate reducer,it can also meet the demand of drilling safely efficiently in deep and ultra-deep wells.