In this talk, recent measurements of charmonium decays of BESIII are presented. Using 448 million $\psi(3686)$ events collected with the BESIII detector, the branching fractions of the decays $\chi_{cJ} \to \phi \phi (J=0,1,2)$ have been measured most precisely, and the polarization parameters of $\chi_{cJ} \to \phi \phi$ have been determined for the first time via a helicity amplitude analysis. Using the same data sample as in the previous study, first evidence of $\eta_c(2S) \to \pi^+ \pi^- \eta$ has been found in the decay sequence $\psi(3686) \to \gamma \eta_c(2S)(\to \pi^+ \pi^- \eta)$. The product of the branching fractions of $\psi(3686) \to \gamma \eta_c(2S)$ and $\eta_c(2S) \to \pi^+ \pi^- \eta$ is reported as well as the individual branching fraction of $\eta_c(2S) \to \pi^+ \pi^- \eta$. The process $e^+ e^- \to \eta J/\psi$ at a center-of-mass energy $3.773~{\rm GeV}$ is observed for the first time. Its Born cross-section is measured, and the branching fraction of $\psi(3770) \to \eta J/\psi$ is determined by a combined fit with the cross sections at other energy points, after considering the interference effect for the first time. Utilizing 2708 million $\psi(3686)$ events collected by the BESIII detector, the decays $\chi_{cJ} \to \Omega^- \bar{\Omega}^+(J=0,1,2)$ have been observed for the first time with high significance via the radiative decays of $\psi(3686) \to \gamma \chi_{cJ}$. The relevant branching fractions have been provided.