The time‐evolving growth behavior during the early phase of an earthquake rupture and when distinguishable features arise for events of different sizes are critical issues for real‐time seismology. These issues are related to the key concept of rupture predictability, which directly determines the warning time and the potential of earthquake early warning systems. P waves carry the temporal information of rupture development, which provides us with an important clue to investigate and understand the source physical process in the early stage of the rupture. In this article, for 36 shallow, moderate‐ to large‐magnitude earthquakes () that occurred in Japan and China, we systematically assess and quantify the temporal evolution of the P‐wave velocity amplitude (V‐env) and the characteristic period (). The initial growth rate and the secondary evolution parameters are extracted to investigate the main features of the early rupture behavior. Our results show that the V‐env exhibits distinguishable growth features after a similar steep rise affected by propagation attenuation for earthquakes of different sizes. Near the distinguishable points, the bell‐shaped fluctuations are generally observed in the curves, and the fluctuation amplitude is positively correlated with the final magnitude. The distinct evolution of P waves radiated from earthquakes of different magnitudes occurs earlier than one‐half of the rupture duration, which constitutes new evidence supporting the weak predictability of ruptures. Finally, we discuss the implications for the physical source mechanism and earthquake early warning.