The new International System of Units (SI) had been implemtented since 20^th 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 analyses of the meaning of the unit’s definitions. This old seven elementary units were still irreplaceable from SI dimension, but they has no more acted as the source of traceability, and their definition has been update. However, the seven elementary constants has already become the root of traceability. Here also give a prospect of the meaning that new SI will develop metrology.