Gas insulated switchgear (GIS) of different manufacturers have different sizes of resin sprues and the size of resin sprue has direct effects on the ultra high frequency (UHF) electromagnetic waves motivated by partial discharge (PD), which brings about many difficulties in accurate estimate of the severity of PD. On the platform of a 252 kV GIS, we used UHF sensors which were mounted on resin sprues of different sizes that had been set up in advance to detect electromagnetic wave signals emitted by a needle-plane PD model in GIS and found that with an increment of the length of resin sprue’s long edge, the signal intensity enhances first and weakens later and reaches its maximum at 75 mm. Another simulation experiment which used pulse signal generator to provide excitation source for the transmitting antenna in GIS to simulate PD and detected signals in the same way to the former also found the same conclusion. This conclusion offers important references for the options of the optimal size of resin sprue and development of external UHF sensor.