  Supernova remnants (SNRs) have been considered as the dominant contributors
to Galactic cosmic rays. However, the relation between high-energy particles
trapped in SNRs and cosmic rays observed at the Earth remains obscure. In this
paper, we fit the spectral energy distributions of 35 SNRs with a simple
one-zone emission model and analyze correlations of model parameters to uncover
the evolution of high-energy particle distribution in SNRs. We find that (1)
the particle distribution in general can be described by a broken power-law
function with a high-energy cutoff for all SNRs; (2) the low-energy spectrum
becomes harder and the break energy decreases with aging of SNRs, (3) for most
middle-age SNRs, the energy loss timescale of electrons at the high-energy
cutoff is approximately equal to the age of the corresponding remnant implying
quenching of very high-energy electron acceleration; for young SNRs, this
energy loss timescale is shorter than the age of SNRs implying continuous
electon acceleration at the cutoff energy; and for a few old age SNRs, the
energy loss timescale is longer than the corresponding age which may suggest
escaping of higher energy particles from SNRs. Finally, we comment on the
implications of these results on the SNR origin of Galactic cosmic rays.
