The measurement on underground medium variation using a repeated artificial source has gradually become another important goal to pursue. In recent years，we developed and improved a technology system with large capacity air-gun excited in land reservoirs by transplanting the marine seismic exploration technology. The excitation effect has close relationship with the capacity of air-gun，water environment，and excitation conditions. As large capacity air-gun must be excited without water environment，we expand the system to downholes. Based on the BHS-2200LL downhole air-gun with the capacity of 250 in3，this paper carries out the characteristics analysis with excitments in 0.2m and 5.0m wells，and the results showed that：(1)The dominant frequency of the air-gun signal excited in 5.0m well is mainly from 10Hz to 40Hz，and lower than that in 0.2m well，which shows that bubble oscillation is better in larger water bodies.(2)In terms of exciting energy，the signal excited in 5.0m well has stronger energy than in 0.2m，and difference of 1 order in magnitude，the signal excited in 5.0m well with a single shot can transmit up to 9km.(3)The air-gun signal has good repeatability at two kinds of borehole. The downhole air-gun exciting technology system has potential application in the dynamic monitoring near fault zone with small scale range，exploration of oil and mineral resources，and modern urban geophysical environment.