In this paper, we are interested in the linear and the nonlinear Rayleigh instability for the gravity-driven incompressible Navier-Stokes equations with Navier-slip boundary conditions around an increasing density profile ρ0(x2) in a slab domain 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>0 is the growth rate in time, g>0 is the gravity constant, k is the wave number and two Navier-slip coefficients ξ± are nonnegative constants. For eachk∈L−1mathbbZ∖{0}, we define a threshold of viscosity coefficient μc(k,Ξ) for the linear instability. So that, in the k-supercritical regime, i.e. μ>μc(k,Ξ), we describe a spectral analysis adapting the operator method initiated by Lafitte-Nguyễn and then prove that there are infinite nontrivial solutions (λn,ϕn) of (0.1)-(0.2) with λn→0 as n→∞ and $\phi_n \in H^4(\mathbf{R}_-) Based on the existence of infinitely many normal modes of the linearized problem, we construct a wide class of initial data to the nonlinear equations, extending the previous framework of Guo-Strauss and of Grenier, to prove the nonlinear Rayleigh-Taylor instability in a high regime of viscosity coefficient