Radio observations of young supernova remnants (SNRs) can shed light on the early evolution of SNRs. We selected G1.9+0.3 which is the youngest SNRs in the Milky Way Galaxy for a study. We compiled the radio flux densities currently available and converted them to the same frequency, which leaves us the evolution of the flux densities for the past nearly 50 years. We found that the flux densities increase before 2008 and decrease afterwards, meaning the flux densities reaching the maximum at an age of about 150--155 yr. We attributed the brightening of the SNR to the increase of either magnetic field or the accelerated high energy electrons. Based on the age at which the flux density reached the peak, combined with the previous numerical simulation, we discussed the ejecta mass of the supernova and kinetic energy released by the supernova explosion.