To address the low efficiency and high cost of current detection methods, as well as the insufficient accumulation of pavement damage data on rural low-grade roads, this study developed a vehicle vibration data acquisition application based on smartphones and proposed a conversion method for calculating an index equivalent to the international roughness index (I_IRI). Vertical acceleration signals collected during vehicle travel were proposed through high-pass filtering and denoising to calculate the vertical displacement, from which a converted IIRI' was derived. Experimental results show that although the data collected by smartphones has slightly lower accuracy than that collected by professional detection equipment, the proposed IIRI' exhibits a high correlation with the traditional IIRI', effectively reflecting pavement roughness characteristics. This method provides an economical, efficient, and convenient solution for roughness detection on low-grade roads, with significant application value for large-scale pavement condition monitoring and preliminary screening.