During the last decade, fiber sensor has drawn extensive attention due to its flexible, insulating, and readily operating in most measurement environment. But generally, fiber sensor is sensitive to more than one environmental parameter at the same time, so the cross sensitivity limits the application of the sensor. In the present work, a novel design scheme of sensing simultaneously temperature and strain via guided acoustic-wave Brillouin scattering is proposed for resolving the cross sensitivity induced by temperature and strain in single mode fibers. In the guided acoustic-wave Brillouin scattering which occurs due to the interaction between two optical co-propagating waves and the transverse acoustic wave in optical fiber, multi spectrum peaks appear when the frequencies of pump and Stokes are appropriate. Brillouin frequency shift is dependent on elastic property of fiber material such as sound velocity, density, Young's modulus, etc. and these elastic properties are influenced by the surroundings. So Brillouin spectrum changes with temperature and strain. Because different acoustic modes of guided acoustic-wave Brillouin scattering have different sensitivities to temperature and strain, characteristic frequencies of different acoustic modes shift at different levels. Then the influences of temperature and strain on elastic property of fiber material, and the relationship between material properties and characteristic frequency of each acoustic mode can be worked out, therefore the temperature and strain can be calculated by the different influences of temperature and strain on each acoustic mode. The simulation results indicate that the temperature sensitivity of R02 mode is 0.86% lower than that of TR25 in the SMF-28 fiber, but the strain sensitivity of R02mode is 54.1% higher than that of TR25. Temperature sensitivity of R02 is approximately equal to that of TR25, but strain sensitivity of R02 is obviously diferent from that of TR25. So the influences of temperature and strain on Brillouin frequency shift can be effectively distinguished, thereby simultaneous measurements of temperature and strain can be realized by guided acoustic-wave Brillouin scattering.