Abstract: Underwater acoustic pressure is the most important acoustic quantity in the research of underwater acoustics. The main electric and acoustics parameters are all calculated by sound pressure. The calibration ofunderwater sound pressure is usually carried out in the free field with a limited water region, in which the reflection from boundaries must be eliminated. In standard facilities, tone burst signal radiation and receiving technique is usually used. A direct signal from a projector and echo signals are divided by time selection method. To suppress random noise, narrow band filtration is used. Time selection method disadvantages are as follows: pulse signal form distortion by transient processes in narrow band path, time selection efficiency decrease along with frequency decrease (as consequence - limitation of free field calibration lower frequency), long time of hydrophone detailed frequency response measurement, need in coherent accumulation for signal-to-noise ration increase. To solve the problem, the paper presents a method to reduce the calibration frequency within limited water region. With the combination of complex linear chirp signals and signal processing method, the target character of the system that is compromised of projectors, water tank and receiver was estimated. The reflective effects were eliminated and the free field was obtained which reduced the low frequency restriction of calibration and satisfied the requirements of sonar calibration in the lower frequency range. The theory of the measurement method and experiments were studied, and good agreements were shown between the results of using this method and free field pulse method.