In China, many highways were built near high mountains where landslide often happened and threatened the traffic safety. Slope monitoring is key issue that could reduce long-term losses and ensure traffic safety on the highways. In this paper, based on the research project sponsored by government, the slope stability monitoring plan was discussed. On the basis of investigation of the art of state methods, synthetic monitoring system was chosen for the landslide early-warning system of national highway 104. It comprised of Time Difference on Arrival monitoring method on the roadside surface as well as inside the underground methods such as borehole inclinometers, piezometers and the earth pressure cells. Then Time Difference on Arrival methodology was focused on, and the location error was analyzed in detail. According to, the theoretic analysis and practical consideration, the optimized arrangement plan of Time Difference on Arrival transducers was proposed. The sketch of all kinds of sensors for the whole system was provided as well, which will ensure the installation of the early-warning system.