In the MEMS fabrication process of 0.345THz micro-electric vacuum FWG-TWT, the periodic FWG slow wave structure is assembled by joining two symmetrical metal pieces with grooves. In ideal design, these grooves’ profiles should be perpendicular to their symmetrical plane while the device’ performance can be best, however those slope angles are actually more than 90 degrees because of their molds’ insufficient exposure. This paper does research on the influence of waveguide’s exposure steepness on the performance of FWG circuit through theoretical analysis and numerical simulation. The range of slope angle studied in this paper is from 0 to 5 degrees, and as the slope angle becomes larger, electromagnetic signal’s transmission attenuation increases. But by adjusting other working parameters of the circuit, the high net gain can be maintained while its bandwidth is going to decrease. These results will be helpful for the experiment.