In this study, we used a new method called the peak‐trough phase shift (PTPS) method to obtain velocity change from the direct surface wave. A graphic user interface was developed to facilitate manual quality control and visualize the results instantly for quality assessment. Comparing the results measured by the stretching method, the PTPS method is proved to be reliable. We applied this method in the Kilauea volcano region and found that the velocity change of the sediment layer is complicated by the joint influences of the precipitation and the eruption. Precipitation‐velocity changes scatter diagrams can help us to quantitatively separate the influences of two factors. The two major contributing factors included the water saturation of microcracks and the filling of magma in large fractures. Significantly, our results revealed two phases of magma activities before the 2018 eruption: (1) in the first phase, the upward migration of the magma increased the compressional stresses and closed the large connected fractures and microcracks and therefore increased velocity, and (2) in the second phase, the magma injected into large veins and fractures, which resulted in sharply decreased velocity.