  In a modified gravity theory, the propagation equation of gravitational waves
will be presented in a non-standard way. Therefore this tenor mode perturbation
of time-space, as a complement to the scalar mode perturbation, provides a
unique character distinguishing modified gravity from general relativity. To
avoid the model-dependent issue, in this paper, we propose a parametrised
modification to the propagation of gravitational waves. We show the effects on
the angular power spectrum of cosmic microwave background radiation due to the
parametrised modification and its degeneracy to the tensor mode power spectrum
index $n_t$ and its running $\alpha_t$. At last, we report the current status
on the detection of modified gravity through the currently available cosmic
observations. Our results show no significant deviation to general relativity.
