Research conclusions ： There are fine relation between the modulus of compressibility and index of the standard penetration test of red clay foundation at Wuhang - Guangzhou Passenger Dedicated Railway, the relative coefficient is bigger than 0. 85. From there relative formula, the modulus of compressibility for the upper strata hard - plastic red clay foundation can be exactly obtained by correcting the modulus of compressibility in geotest indoor through the number of hammer in standard penetration test. In addition，the amount of drilling and sample geotest indoor can be reduced，and such problem as the numerical value on the low side in test indoor by the unqualified sample of commom drilling and sampling measure can be greatly reduced.