In this paper, the stimulated Brillouin scattering (SBS) threshold of step-index few-mode fibers (FMFs) is theoretically analyzed. The scattering characteristics of FMFs are analyzed using the mathematical model of the Brillouin scattering spectrum. The gain spectrum of Brillouin scattering and the influence of fiber parameters on the SBS threshold of FMFs are discussed, and the SBS theoretical threshold is obtained according to the parameters of FMFs. A new experimental system for measurement of the SBS threshold of FMFs based on two photonic lanterns is built that can eliminate the Fresnel reflection. The measured SBS thresholds of the LP01 and LP11 modes over 9.8 km two-mode fiber are 10.93 dBm and 11.35 dBm, respectively, and the measured SBS thresholds of LP01, LP11, LP21, and LP02 over 11 km four-mode fiber are 10.18, 10.38, 11.71, and 12.23 dBm, respectively. The experimental results agree well with the theoretical analysis, and the deviation is less than 0.7 dB. The theoretical analysis has guiding significance for setting the incident power in FMF communication and sensing. The experimental system can accurately measure the SBS thresholds of FMFs.