The design and tests of a photon shower position detector are discussed in this thesis. Some Monte Carlo results about this detector are also discussed. It will be used to measure the shower position of photons so that single γ QED events can be distinguished from the other kinds of events in γp interaction. The results are encouraging. From the Monte Carlo results, the detector should do a decent job of picking up QED events. The QED event ratio will be around 40% of all events which pass our cuts. And from the offline cosmic ray test and limited online test data, the detector will function very well.