Abstract: Different partially hydrolytic products of polymethyl methacrylate(HPMMA) with relatively high molecular weight can reduce effectively the water loss of the sea water-base mud at temperature high up to 180℃.The quantities used vary from 1.5% to 3% depending upon the molecular weight and the degree of hydrolysis of the polymer.It was found that HPMMA suffered from thermal degradation under 180℃,and its stability to temperature can be increased considerably when it was cross-linked with ethanolamine or was added to the mud together with a small amount of antioxidant. In order to have a preliminary investigation on the mechanism of water loss reduction of sea water-base mud by HPMMA,the rheological properties of muds treated with different HPMMA in different amounts were observed with a rotation viseometer and the dispersity of clay particles in the muds and the structure of the filter cakes were studied by scanning electron microscope and sedimentation analysis.A physical model has been proposed.