Microstructure and mechanical property of 2099 alloy under different pre-stretch degree have been studied by micro-hardening test, mechanical test and TEM observation. The results show that with the increase of pre-stretch the aging process is accelerated greatly and the mechanical properties are improved obviously. Precipitation phases become more fine and dispersed in matrix, some coarse phases in grain boundaries tend to discontinuous and point chain. The main aging strengthen phases of 2099 alloy experience a transformation from T1, δ′ and θ′ three phases coexistence to T1 phase, which indicated that pre-stretch have important effect on the size, type and distribution of precipitation, which can promote T1 phase precipitation in the expense of δ′ and θ′ phases consumption during the aging process.