On the basis of the P-S wave reflection coefficient equation given by Aki & Richards for weak contrast media, a new approximation formula for the small incident angle is presented. For the purpose of formulating the new equation, binomial expansion is applied to the factors related to incident and transmission angles, and the sine of the incident angle is replaced by incident angle directly. Thanks to its simple form and few elastic parameters, the new equation can be easily employed to the parameter inversion. Calculation results on different models show that it is equivalent to the original equation by Aki & Richards when the incident angle is small and can make some improvement when the angle is somewhat larger. In addition, the first order approximation form can invert the shear wave impedance.