Abstract: A circuit for detecting arcing in rectangular waveguide is introduced, and the selection of arcing detecting point and photosensitive instruments are analyzed. The conclusions of theoretical analysis and calculations are that the arcing detecting point is chosen in the middle of the rectangular waveguide narrow walls and the photoelectric cell SFH250V is utilized as the photosensitive instrument. Treating SFH250V as the key instrument, an arcing detecting and malfunction fast flip-latch circuit is designed, which solves the problems of weak optical signals detection and weak photocurrent signals conversion and amplification. Meanwhile, the circuit of the arcing malfunction estimation and malfunction signals export is designed. The experimental results indicate that this setup can catch weak arc malfunction signals efficiently.