The Gaussian laser beam is shaped into a flattop laser beam with uniform energy distribution by a diffractive optical element. The Cu plasma characteristics induced by two lasers are compared, and the improvement of the beam shape modification on the stability of laser-induced breakdown spectrum is studied. The research results show that the uniformity of beam energy distribution causes the difference in the morphology of the ablation crater and single-shot ablation amount. The relative standard deviations of the Gaussian laser and the flattop laser induced spectra intensity, plasma temperature and electron density are 12.33% and 6.37%, 2.10% and 1.32%, 5.31% and 0.65%, respectively. The stability of the laser-induced breakdown spectrum after beam shaping is significantly improved, and the two laser-induced plasmas are all in the local thermodynamic equilibrium state.