A solar flare is a sudden, rapid, and intense variation in brightness as observed at the Sun. It typically associates with converting magnetic energy to kinetic energy in the form of solar energetic particles (SEPs). The objective of this work is to study the propagation of SEPs from the Sun to the Earth. We simulate the particle propagation for the solar event on August 9, 2011 by using the transport equation from Ruffolo 1995. We solve the transport equation by the numerical technique of finite different method. We find injection duration by fitting the simulation results and the particle data from spacecraft. The X-ray class of the selected solar event is X6.9, the solar flare position on the Sun is N18W68, and the solar wind speed is 551.5 km/s. We found the solar flare on August 9, 2011 is the gradual flare. This flare had the long injection time from the Sun to the Earth corresponding to the shock wave detected after explosion in the interplanetary space 13 minutes. In the path of the solar flare affected on the Earth, the Kp-index (the value of the earth’s magnetic field variable) was considered. The Kp-index of these solar flares was less than 3, which they didn’t affect on the Earth.