In sheet metal forming, friction condition between sheet metal and mold is a very important problem for it influences the formability and the variation of stress, strain and thickness of sheet metal, and so on. Simulation experiments had been done to measure friction coefficients online at punch and die rounded corners under different process parameters during warm deep forming of magnesium alloy sheet. The results made it clear: temperature shows the greatest effect on friction coefficients, the effect of blank holding force (BHF) secondly, and no significant effect of lubricant on the friction coefficients. The friction coefficients at die rounded corner are larger than those at punch rounded corner. The dynamic changed friction coefficients models with temperature and BHF were fitted by the experimental data respectively, and they were added into simulation software through further development by user subroutine VFRIC using FORTRAN language to improve the accuracy of the numerical simulation.