The new international system of units (SI) had been implemented since 20th May 2019, that has epoch making magnificent meaning. This review includes each developing process to seven constants from the seven elementary units of Second, Meter, Kilogram, Ampere, Kelvin, Candela, Mole and revealed intrinsic relationship between them, and gave the deep analysis of the meaning of the unit’s definitions. This old seven elementary units were still irreplaceable from SI dimension, but they have no more acted as the source of traceability, and their definition has been updated. However, the seven elementary constants have already become the root of traceability. Here also gives a prospect of the meaning that new SI will develop metrology.