The effect of pre-stretching on microstructure， mechanical properties and strength contribution of 2195-T8 Al-Li alloy with 21 mm plate thickness was investigated. The results show that， the diameter of T1 precipitates decrease and their number density increase with increasing the pre-stretching； while the diameter and the number density of θ′ precipitates decrease. The yield strength increases from 596 MPa to 638 MPa with increasing pre-stretch from 3% to 15%， but the elongation decreases from 13.8% to 10.7% when aged at 148 ℃ for 38 h. With increasing the pre-stretch， the contribution of precipitation strengthening to yield decreases， and the contribution of work hardening increases. The yield strength depends on the combined effect of work hardening and precipitation strengthening.