In this study

we studied the surface subsidence law and the shortcomings of predicting surface subsidence by Knothe time function model under loose formation conditions. To express the loose formation subsidence

a time impact factor c2 is introduced into the original time

and a two-factor time function model is developed. Theoretical analysis results indicate that although the two-factor time function has characteristics of slow convergence and strong adaptability to parameter change

it is not complete in accordance with the dynamic process of the surface subsidence. Hence

we improve the two-factor time function，and then verify the applicability of improved two-factor time function for three different thickness (i.e.

thin

thick and extremely thick) of loose strata. The results show that the improved function can predict the whole dynamic process of ground point subsidence under the conditions of different thickness of loose strata. The results performed in the study can provide reference for coal mining under building

railway and water body.