The high integration and high energy density of the RF front-end make the nonlinearity problem in surface acoustic wave (SAW) devices more and more serious. A measurement system of nonlinear signals in RF SAW devices has been set up in this work, and the second harmonic (H2) and third order harmonic (H3) signals of SAW resonators are measured precisely. The measured results are analyzed and the generation mechanisms of nonlinear harmonic signals are discussed. By comparing the fitted simulation result of nonlinear finite element method (FEM) model with the measured results, the influences of the dielectric nonlinearity and the nonlinear effect of acoustic strain on the harmonic generation are verified, which provides an important support for further study on the nonlinear suppression method and design of SAW devices with high linearity.