Abstract: Instability of drilling string has a number adverse impacts on drilling operation such as difficulty in weight transfer on bit, tendency of pipe sticking and poor control of borehole wall stability and borehole trajectory. A test of drilling string stability was conducted by Petroleum University on the test apparatus for drilling string stability designed by the horizontal drilling research group. During test, the borehole wall was simulated by transparent plexiglass pipe with I.D. 30mm and O.D. 40mm,and the drill pipes under compression with various O.D.,stiffness and weight per unit length were simulated by red copper tube of respective size:?14mm, ?12mm, ?10mm and ?8mm. Various Specimens were tested under inclination angles of 90°, 6°, 30° and 0°. The results showed that:1. Generally specimens exhibited similar behavior, i.e. in three stages such as non——insatiable status, initial insatiable and quasi——spiral insatiable stable status, and quasi——spiral insatiable and spiral Insatiable status. 2. Drag has a hysteresis effect to drilling string instability, which can enhance its stability to a certain extent. 3. By changing the Inclination of specimen, the instable load on specimen increases as the angle with vertical direction increases due to specimen’s weight. which Indicates the consistency with theoretical analysis. From the test result it can be seen that most the existing theoretical prediction are higher than test results except that the quasi-spiral theoretical results approach the test results which can be regarded as reference for drilling practice.