The weathered lamprophyre is unstable water-sensitive formation, wall collapse often occurs in drilling process with drilling tools sticking and burying. According to the characteristics of weathered lamprophyre formations and the problems encountered in drilling, experimental study was made on wall protection technology with flushing fluid in weathered lamprophyre formation, and PHP-CORESMART solid-free flushing fluid was developed. The flushing fluid is of strong inhibition and film forming ability being prepared with PHP, Na-CMC and CORESMART, which is applied successfully in weathered lamprophyre layer.