As the working frequency of a vacuum electron device reaches the terahertz frequency band, the cross section of the surface wave oscillators (SWO) becomes very small, and the micro-fabrication precision of the device cannot be guaranteed, at the same time, because the electromagnetic field of SWO concentrates on the inner surface in slow wave structure, when the working voltage of surface wave oscillator is very high, the explosive emission probability of the slow wave structure increases greatly, and the explosive emission can influence the working characteristic of the device. This paper analyses the distributing property of the electrical field in the slow wave structure of 0.14 THz SWO. Parameters of the SWO under study are as follows: working voltage is 312 kV, explosive emitted current is 1.67 kA, periodic length of the slow wave structure is 0.7 mm, width of the slot is 0.4 mm, and the height of it is 0.3 mm; cold-test results indicate that the amplitude of the electrical field in the slow wave structure varies sinusoidally; the amplitude of the electrical field reaches a maximum value in the middle of the slow wave structure near its inner surface, and the explosive electron emission can occur most possibly in this position, because the electrical field in the slow wave structure varies with very high working frequency. The explosive emitted electron may bombard back the slow wave structure, and the secondary electrons will be emitted at a certain probability, for which the formula proposed by Vaughan is used to compute the secondary emission yield, and this formula is implemented in the self-developed particle-in-cell code UNIPIC; while the code is used to simulate 0.14 THz SWO with explosive emission in the slow wave structure. In the simulation, the slow wave structure multipactor discharge induced by electrons is also considered; the phase space of the electrons emitted from the slow wave structure shows that the energy of secondary electrons is below 5 keV, so the validity for secondary electron yield is affirmed. Numerical simulation results indicate that because the emitted electrons from the slow wave structure change the distribution character of the electrical field in the slow wave structure, especially the amplitude of the electrical field in the middle of the slow wave structure, the beam-wave interaction is weakened, and as a result, output power decreases from about 22.6 megawatts to only 1.89 megawatts.