[en] A new 3.7GHz/2MW/2s LHW system was built on HL-2A tokamak, where TH2103A type klystrons were used as the high power long pulse wave sources. The magnet field distribution in central axis for the klystron was measured via a device for probe fixing. The strongest magnetic field was 176mT at the rough middle of the magnet. Each coil current separately plays the most important role in magnet field distribution at different position. The influence of No.1 power supply was strong nearby the cathode and weaker nearby the resonant cavity. The influence of No.2 power supply was strong nearby the resonant cavity and weaker nearby the output window. The influence of No.3 power supply was strong nearby the output window but much weaker at other places. The data obtained supported for klystron operation optimization. (authors)