Study on the precursor infrasound waves emitted before the occurrence of strong earthquakes has been performed, so as to discover the relationship among position, arriving time, intensity of the infrasound wave and earthquakes. With the special kind of infrasound microphone CASI-ICM-2011, a kind of infrasound waves with frequencies from 0.001 Hz to 0.01 Hz, peak sound pressure level from 50 Pa to 200 Pa, continuous time period from half hour to 4 hours, and propagation speed from 10 m/s to 30 m/s, arising no more than two weeks, was received before a series of earthquakes over Ms6.0 occur. Amplitude of the signal is higher when the earthquake is stronger. In the far field the sensor network was spread in north-east China with automatic data uploading to a central server computer in Beijing. Precursor infrasound waves emitted 4 days before Ms7.0 Lu-san earthquake have been positioned perfectly as a sound cloud map, also the infrasound wave emitted 12 days before Ms7.7 Pakistan earthquake has been positioned. A long-time continuous signal over 8 years has been analysed without a bit gap, showing several effective signals accompanying earthquakes. Law of the infrasound generation has been discussed with a suggestion for the mechanism that the infrasound could be radiated by a large scale surface vibration near one million square kilometers in earthquake developing. Two demonstrative signals received after Ms8.8 Chile earthquake and Yu-shu earthquake were provided to prove this suggestion. The detected signal shows that the infrasound wave arrives accompanying S wave at the same time. One model is provided as a piston sound source to simulate very low frequency infrasound radiated by large surface vibration. All the presented signals in this paper should be useful for precursor information obtained for close earthquake prediction.