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More available warning time and a quicker rapid intensity reporting after earthquake are of great significance to the earthquake early warning system and the after earthquake rescue. The improvement of the calculation speed of the instrument intensity and the accuracy of the calculation results are very important for the whole early warning process. The development of intensity in Japan has gone through three stages：instrument intensity of JMA in 1996，real-time instrument intensity in 2008 and real-time instrument intensity in 2013. This paper explains these three and gets the contract picture by using the data in the 2011 off the pacific coast of Tohoku earthquake. The instrument intensity using 2013 method is quicker than that using 1996 method，and the instrument intensity using 2013 method is closer to that using 1996 method，compared with that using 2008 method.