Decomposing functional connectome to low-dimensional spaces revealed a principal gradient cross brain hierarchy from the primary sensorimotor areas to the default mode network. However, the physiological meaning underlying this principal gradient remains unclear. Here, we showed significant propagating activities of resting-state fMRI along this gradient direction. We further used simulation to demonstrate that the propagating activity can result in a gradient in its propagating direction with the use of the low-dimensional embedding method. Overall, the findings suggest that the principal connectivity gradient actually represents the major propagating direction of spontaneous brain activity, which is likely across brain hierarchies.