A new experimental method for on-resonant two-photon absorption speotroscopy is presented in this paper. The experimental set-up is the same as that of the two-photon fluorescence experiment used in measuring durations of ultrashort laser pulses. A picosecond laser pulse is divided into two beams which then travel in opposite directions in the sample. The signal intensity measured along the laser trace will be a wide plateau overlapped by a narrow peak. The ratio of the on-resonant two-photon absorption section to the two-step single-photon absorption section can be deduced from the contrast ratio of the peak intensity to the shoulder intensity. The theo xetical calculation shows that this method is valid for various oases.