In the functional performance tests (FTP) of Dianxibei UHVDC project, after 700 ms two-phase metal short-circuit fault on AC line at the rectifier side, it is found that valve misfire protection misoperation leads to instant outage of pole 2 in Xinsong Station. In this paper, the cause of valve misfire protection misoperation is analyzed theoretically. It is found that the AC voltage waveform distortion is caused by the interaction between the wave trap coil and the lightning arrester after the unbalanced fault at the AC side of the rectifier station of the Dianxibei UHVDC project. The original low AC voltage criterion of valve misfire protection cannot match the condition and causes misoperation. Finally, two solutions are proposed in which the true RMS and the fundamental component of the AC voltage are used to judge low AC voltage, and the effectiveness is verified.