Abstract: From the accident data, the main safety problem associated with highway medium guardrail in China now is the lack of the strength of the post抯 base soil. The effort in this paper is concentrated on the study of stability of the guardrail post during collision process of uncontrolled cars with the guardrail structure. An analysis of fencepost in clay base is given. The situation in backfilled clay base is compared with that of the compacted, and a reasonable reinforce scheme is given.