This paper reports a new drilling fluid system--positive sol drilling fluid (PSDF). The electrokinetic behaviour, rheological properties and the ability to inhibite clay swelling of positive sol (PS) and PSDF were studied. It was found that PSDF might form weak negative system and positive system under different conditions, and the gel strength and rheological parameters of PSDF could also be adjusted to meet drilling technology demands by changing the ratio of clay-sol. Both PS and PSDF demonstrate high inhibition ability, and PSDF can be treated with conventional additives. In this paper, the field test results of PSDF are introduced as well.