Convolution integral and convolution sum are principal methods in the linear time invariant systems analysis. At present, many text books that have been published in the my home country or foreign country, especially the Signals and Systems writing by Oppenheim A V and Signals and Systems writing by Haykin Simon, introduced the methods by use of the graph to determine the up limit, low limit and the exist interval of the integral or convolution sum. The closed methods that evaluate the convolution integral and convolution sum avoid to determine the up, low limits and the duration of the convolution integral and convolution sum by means of the graph，So that, the convolution integral and convolution sum according to the definition can be evaluated directly.