In this work, we study the nonlinear spatially homogeneous Landau equation with Maxwellian molecules, by using the spectral analysis, we show that the non linear Landau operators is almost linear, and we prove the existence of weak solution for the Cauchy problem with the initial datum belonging to Shubin space of negative index which conatins the probability measures. Based on this spectral decomposition, we prove also that the Cauchy problem enjoys S 1 2 1 2 -Gelfand-Shilov smoothing effect, meaning that the weak solution of the Cauchy problem with Shubin class initial datum is ultra-analytics and exponential decay for any positive time.We shall study the linearization of the Landau equation (1.1) near the absolute Maxwellian distribution µ(v) = (2π) − 3 2 e − |v| 2 2 . Considering the fluctuation of density distribution function1 since Q L (µ, µ) = 0, the Cauchy problem (1.1) is reduced to the Cauchy problemand the Shubin spaces, for β ∈ R, (see [15], Ch. IV, 25.3),We havewhere H β (R 3 ) is the usual Sobolev spaces. In particular, for β < − 3 2 the Shubin space Q β (R 3 ) contains the probability mesures (see [2, 9, 12, 17] and [8]). See Appendix 6 for more properties of Gelfand-Shilov spaces and the Shubin spaces. 2