In this paper, we are interested in the linear and the nonlinear Rayleigh instability for the incompressible Navier-Stokes equations with Navier-slip boundary conditions around a laminar smooth density profile ρ0(x2) being increasing in an infinite slab 2πLT×(−1,1) (L>0, T is the usual 1D torus). The linear instability study of the viscous Rayleigh-Taylor model amounts to the study of the following ordinary differential equation on the finite interval (−1,1), \begin{equation}\label{EqMain}-\lambda^2 [ \rho_0 k^2 \phi - (\rho_0 \phi')'] = \lambda \mu (\phi^{(4)} - 2k^2 \phi'' + k^4 \phi) - gk^2 \rho_0'\phi,\end{equation} with the boundary conditions \begin{equation}\label{4thBound}\begin{cases}\phi(-1)=\phi(1)=0,\\\mu \phi''(1) = \xi_+ \phi'(1), \\\mu \phi''(-1) =- \xi_- \phi'(-1),\end{cases}\end{equation}where λ is the growth rate in time, k is the wave number transverse to the density profile and two Navier-slip coefficients ξ± are nonnegative constants. For each k∈L−1Z∖{0}, we define a threshold of viscosity coefficient μc(k,Ξ) for linear instability. So that, in the k-supercritical regime, i.e. μ>μc(k,Ξ), we provide a spectral analysis adapting the operator method of Lafitte-Nguyễn and then prove that there are infinite solutions of \eqref{EqMain}-\eqref{4thBound}. Secondly, we will extend a result of Grenier, by considering a wider class of initial data to the nonlinear perturbation problem, based on infinitely unstable modes of the linearized problem and we will prove nonlinear Rayleigh-Taylor instability in a high regime of viscosity coefficient, namely μ>3supk∈L−1Z∖{0}μc(k,Ξ)