Case-Based Reasoning (CBR) is a most promising direction in artificial intelligence field. To bring actual case reasoning tech into drilling fluid formula optimal design, and to join the actual course, it is the first time to adopt a mixed searching strategy that involves knowledge guided method, nearest neighbor method and improved range similarity match method. To be exact, first to find the similar drilling fluid type, then to modify its performance parameter value till maximum optimized and rematch, finally get the most optimal drilling fluid formula. Such design method can largely improve the efficiency and accuracy of drilling fluid formula.