Abstract: The design of rock bedded cutting slope and its support is one of the difficult problems in foundation design of railway and highway. The unstable rockmass on the bedrock with a high dip angle may be dug out. But it may be very expensive to dig all rockmass on the bedrock with medium and low dip angle.So the extent of unstable rockmass is of great concerned.According to the need of demonstration and design of the railway from Chongqing to Huaihua, the formulae to calculate the length of unstable rockmass of the bedrock slope were deduced. The comparison between the calculated and observed data was carried out.It shows that the results are coincident.