But we know that atom like lower energy state so whenever it will go higher energy state (when absorbtion spectrum will take place) it should come to lower energy state(emission spectrum should take place).what i am understaning is it should be simultanous process ?

Yes, the atoms absorb the energy and its electron jumps from lower orbit to higher orbit. The electron tends to be in the lowest energy state and hence it jumps immediately to the lower energy state.

The process seems simultaneous however, there would be a time difference of the order of nano seconds.

Hence, for an observer, the absorption and emission spectra are simultaneous.