Earthquake early warning (EEW) is one of the effective ways to reduce earthquake damage. Earthquake magnitude estimation is the most important and also most difficult part in the whole EEW system. Based on 142 earthquake records from KiK-net in Japan and aftershock records of the great Wenchuan earthquake in Sichuan, China, we worked out the magnitude estimation relationships by using tau;c and Pd methods, with corresponding variances of 0.62 and 0.56, respectively. In order to remove the effect of magnitude saturation, we expanded the Pd method and proposed a new method of continuously calculating the peak displacement. When the time window is 10 s, the variance of magnitude estimation is reduced to 0.37, fully meeting the need of EEW systems. Meanwhile, our method realized the continuous transition of different information, and improved the utilization of seismic records. We also discussed the relation between Pd and PGV, and the compatibility between different characteristic parameters.