  The nonleptonic two-body $\Lambda_b\rightarrow \Lambda\psi $ decays with
$\psi=J/\psi$ or $\psi(2S)$ are investigated based on the perturbative QCD
approach. These are color-suppressed processes in which the nonfactorizable
contributions are confirmed to be dominant. Angular momentum conservation
allows us to describe the concerned decays by four independent complex helicity
amplitudes. It is observed that the negative-helicity states for the $\Lambda$
baryon are preferred as expected in the left-handed nature of the
charged-current interaction. The obtained results for the helicity amplitudes
are used to compute the branching ratios and various observable parameters,
which are then compared to the existing theoretical predictions and
experimental data. In particular, we predict the ratio
$\mathcal{R}=\frac{\mathcal{B}(\Lambda_b\rightarrow\Lambda
\psi(2S))}{\mathcal{B}(\Lambda_b\rightarrow\Lambda
J/\psi)}=0.47^{+0.02}_{-0.00}$ in comparison with $0.508\pm 0.023$ from the
Particle Data Group at the level of 1 standard deviation. We also briefly
explore the long-distance contributions to the semileptonic $\Lambda_b
\rightarrow \Lambda l^+l^-$ decays in the kinematic regions where the dilepton
invariant masses are around the $J/\psi$ and $\psi(2S)$ resonances.
