In order to get pavement structure modulus which can real reflect pavement mechanical characteristics and the properties of pavement materials，four kinds of full-scale unbound aggregate base asphalt pavement test road are constructed，accelerated pavement testing is conducted and pavement deflection is tested. Based on FWD testing program，layered measure and analysis method are introduced to measure deflections，FWD deflection testing is conducted in different stages of pavement damage. The influence characteristics of loading level，position and pavement temperature on FWD backcalculated modulus are analyzed，the relationship between FWD backcalculated modulus and laboratory dynamic modulus is established，and the conversion coefficients of asphalt concrete，unbound aggregate and subgrade soil are put forward. The results show that the asphalt concrete modulus is greatly affected by temperature and wheel loads，asphalt layer backcalculated modulus conversion coefficients is of 0.5 to 1.0 if the pavement is in good condition，and it is of 1.3 to 1.5 if pavement top-down cracking is occurred. If the pavement is damaged at the bottom of the asphalt layer，the conversion coeefient is more than 1.5. The conversion coefficient of unbound aggregate is of 1.32 to 1.49；the subgrade soil backcalculated modulus conversion coefficient is related with stress level and soil types，the conversion coefficient of inverse modulus for soil base decreases with decreasing stress.