The low velocity zone is a mechanical weak band in the crust，which plays an important role in breeding of earthquakes． Artifical detection and nature earthquake tomography both show that low velocity layers exist in different depths in North China． We analyzed the highresolution data detected by large volume airgun source in North China in order to put further constraint on the properties of the low velocity layers there． We found the following main characteristics in airgun data: firstly，apparent velocity of the direct P-wave’s is approximately 6km / s，which could be traced upto nearly 70km; secondly，the PmP phase which reflected from Moho begin to appear as strong phases from 50km; thirdly，the PmP phase at 150km is 2s lag of the Pg phase． All these features show that the low velocity layer exist in the crust in this area． We use synthetic seismograms to forward modeling waveform to fit the observed data． The results showed that synthetic seismograms can not only explain the travel time of various seismic phases， but also obtain the amplitude ratio of PmP and Pg． The results of our modeling include: ① the average thickness of crust in this area is 33km; ② there exists a low velocity layer in both the upper and lower crust in this region． By comparing the heat and magnetotelluric data，we conclude that the low velocity layers in the upper and lower crusts were caused by fluid and thermal effect respectively．