Abstract: Based on the compaction model of sandstone-mudstone alternating beds, we advance a new formula for formation grain density, which conforms to grain volume and mass balance law. Based on the porosity-depth function and the density-depth function, we can use the new formula to calculate the grain density of formation upon the basement. The calculating result can be used to replace the paleo-formation grain density to calculate the paleo-formation average density (ρ_s) which is very important to tectonic subsidence.The traditional formula indicated that sandstone percent is the only affecting factor to the formation grain density. The new formula hints that formation grain density is affected by sandstone percent, average sandstone porosity and average mudstone porosity. Further, the decisive factors to the formation grain density are average density and average porosity of the formation.