Mode analysis of the terahertz wave generated by 0.14 THz overmoded surface wave oscillator (SWO) (overmoded ratio D/ 3) is theoretically accomplished and experimentally validated. At first, longitudinal field-expansion method for mode analysis is established aiming at overmoded Cherenkov devices. Then this method is used to analyze the theoretical mode content of 0.14 THz SWO in detail based on the simulation results of electric field extracted from a 2.5-dimensional PIC (particle-in-cell) code. Results show that the mode content of terahertz wave in different characteristic regions of the oscillator is varied due to the mode conversion, and it is dominated by TM02 and TM03 modes accompanied by a small quantity of TM04 mode at the output. Finally, the energy distribution in the near-field radiation of 0.14 THz SWO is obtained by image-displaying method. The experimental result is reasonably in accord with the theoretical distribution calculated from the mode analysis results under experimental conditions, testifying the feasibility of longitudinal field-expansion method for the mode analysis and the correctness of its results.