The [2+2] photocycloaddtion of 7-methacryloyloxy-4-methylcoumarin attached to the PMMA chain can transfer a semidilute solution to a homogeneous and speckle-free chemical gel with some "extraordinary" dynamics. First, its normalized intermediate scattering function f(q,tau) can fully relax to zero, indicating that the gel has no frozen-in static component. This supports our previous conclusion that the speckles of polymer gels originates from large voids inside, not cross-linked chains (clusters). Second, f(q,tau) consists of a fast and a slow relaxation. Both of them are nondiffusive. For the fast mode, there is nearly no change in its decay rate as well as its related scattering intensity during the sol-gel transition, but for the slow mode, the relaxation slows down and its related scattering intensity sharply increases similar to10(2) times. The alternative analysis of the measured time correlation function by the partial heterodyne method leads to only one relaxation rate, similar to that of the slow mode. Our results confirm that the fast mode is related to the well-known motions of subchains (blobs) between two cross-linked points and reveal that the slow relaxation is due to thermally agitated density fluctuation of the gel network. In comparison with its corresponding semidilute solution, the cross-linking makes the motions of different blobs more correlated, and at the same time, reduces the dimension (static correlation length) of the density fluctuation and slows down its relaxation rate.