Abstract: Using Gleeble-1500 welding thermal simulation test machine, the effect of cooling time from 800~500℃ (t_8/5) on the microstructure and mechanical properties of HAZ of low-alloy austempered ductile iron welding has been studied under the fixed heating rate (150℃/s), peak temperature (1050℃) and peak temperature holding time (7s). The experimental results show that when t_8/5=9~15s, the amount of high carbon martensite is more than 95% in the matrix structure. The tensile strength (σ_b) and impact toughness (α_K) sharply decrease. When t_8/5=20~30s, the martensite and troosite coexist. With the increasing troosite amount, the σ_b and α_K increase. When t_8/5=40s, martensite disappeared and the matrix is consisted of 97% troosite, its σ_s=629 MPa, δ=4.5% and α_K=28.5 J/cm². Further increasing t_8/5 up to 100~200s, some sorbite and ferrite appear, the σ_b and α_K slightly decrease.