Non-linear growth of density fluctuation in one-dimensional self-gravitating system in the expanding universe is investigated. It is found that the Fourier spectrum of density fluctuation obeys a power law in the small-scale when the singularity of density is first formed. The power index of the Fourier spectrum is -1/3 for almost any initial conditions of density perturbations. The power index is determined only by the type of singularity of the density, which is A3 type in accord with the classification in the catastrophe theory. We also found thatthe effect of non-linearity is very large for the growth of density fluctuations compared with the estimate of the growth rate using a linear theory.