Using a three-dimensional test particle simulation code, we study the pondermotive scattering of an ultrashort electron beam subject to a focused ultraintense femtosecond laser pulse. The dependence of the spatial distribution of electron beam and electron energy spectrum on the time delay between laser pulse and electron pulse is discussed in detail. Through the measurement of the spatial distribution of electron beam or electron spectrum, we present a method to measure the duration of an ultrashort electron pulse.