Static and dynamic nonlinear elastic experiments have clearly revealed that rocks have strong nonlinear elastic characteristics. The dynamic method that used resonance to load stable strains inside the rock has achieved good results. However, it’s difficult to perform the experiment in the field. We used propagating longitudinal waves to load strain inside the rock and developed a copropagating longitudinalacoustic waves method to study the nonlinear elastic characteristics of rocks. This method loads a strain inside the rock through the propagationof low- frequency and high- amplitude longitudinal waves and measureschanges inthe nonlinear elastic modulus in the rock through the propagation of relatively high- frequency and low- amplitude longitudinal waves at the same time. The change innonlinear elastic modulus can certainlyreflect the nonlinear elastic characteristics of the rock. When the measurement sample is a linear elastic material, the change of its nonlinear elastic modulus is usually zero; when the measurement sample is a rock, the change of its nonlinear elastic modulus shows a certain regularity.