  In this paper, a time variable cosmological constant (CC) from
renormalization group equations (RGEs) is explored, where the renormalization
scale $\mu^2=R^{-2}_{CC}=Max(\dot{H}+2H^2,-\dot{H})$ is taken. The cosmological
parameters, such as dimensionless energy density, deceleration parameter and
effective equation of state of CC etc, are derived. Also, the cosmic
observational constraints are implemented to test the model's consistence. The
results show that it is compatible with cosmic data. So, it would be a viable
dark energy model.
