Estimating the magnitude based on P waves is important to issue early messages to the general public for earthquake early warning (EEW) purposes. First, we reviewed part of the magnitude estimation results of the current EEW system deployed in Yunnan and found that the magnitude of the mainshock of the Yangbi earthquake sequence (Ms 6.4, as formally determined by the Yunnan Seismic Network) is severe underestimated by 0.9, whereas a number of small earthquakes are overestimated. Then, we determined the reason for this phenomenon and improved the magnitude estimation precision in Yunnan by introducing the fixed effect regression technique, adopting a flexible time window and deriving the magnitude estimation equation strictly. Finally, we examined the deficiencies of this magnitude estimation approach and demonstrated that it is possible to provide instant and reliable parameters of an earthquake through numerous inexpensive accelerometers on the ground.