The traveltime curves of P-S reflections are not hyperbolic. Thus, the classical hyperbolic approximation may not be valid for converted-wave data processing. This paper presents a new method, which expands the t² -x² formula with only two terms because a higher-order Taylor series expansion is mathematically complicated and also becomes inaccurate with increasing offset. The coefficient of the second term can be calculated accurately through explicit equations. The second term can be also factorized into a squared constant and a squared variable, which amounts to a transformation of velocity and offset. In the transformed system, the traveltime curves are hyperbolic and conventional processing procedures can be carried out. Synthetic stacks indicate that this method is valid until the percentage error of V_P estimation is out of the range of -5% to 10%.